This reverts commit c7070619.

This has been shown to regress on some ARM systems:

by forcing on DMA API usage for ARM systems, we have inadvertently
kicked open a hornets' nest in terms of cache-coherency. Namely that
unless the virtio device is explicitly described as capable of coherent
DMA by firmware, the DMA APIs on ARM and other DT-based platforms will
assume it is non-coherent. This turns out to cause a big problem for the
likes of QEMU and kvmtool, which generate virtio-mmio devices in their
guest DTs but neglect to add the often-overlooked "dma-coherent"
property; as a result, we end up with the guest making non-cacheable
accesses to the vring, the host doing so cacheably, both talking past
each other and things going horribly wrong.

We are working on a safer work-around.

Fixes: c7070619 ("vring: Force use of DMA API for ARM-based systems with legacy devices")
Reported-by: NRobin Murphy <robin.murphy@arm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Acked-by: NMarc Zyngier <marc.zyngier@arm.com>

Booting Linux on an ARM fastmodel containing an SMMU emulation results
in an unexpected I/O page fault from the legacy virtio-blk PCI device:

[    1.211721] arm-smmu-v3 2b400000.smmu: event 0x10 received:
[    1.211800] arm-smmu-v3 2b400000.smmu:	0x00000000fffff010
[    1.211880] arm-smmu-v3 2b400000.smmu:	0x0000020800000000
[    1.211959] arm-smmu-v3 2b400000.smmu:	0x00000008fa081002
[    1.212075] arm-smmu-v3 2b400000.smmu:	0x0000000000000000
[    1.212155] arm-smmu-v3 2b400000.smmu: event 0x10 received:
[    1.212234] arm-smmu-v3 2b400000.smmu:	0x00000000fffff010
[    1.212314] arm-smmu-v3 2b400000.smmu:	0x0000020800000000
[    1.212394] arm-smmu-v3 2b400000.smmu:	0x00000008fa081000
[    1.212471] arm-smmu-v3 2b400000.smmu:	0x0000000000000000

<system hangs failing to read partition table>

This is because the legacy virtio-blk device is behind an SMMU, so we
have consequently swizzled its DMA ops and configured the SMMU to
translate accesses. This then requires the vring code to use the DMA API
to establish translations, otherwise all transactions will result in
fatal faults and termination.

Given that ARM-based systems only see an SMMU if one is really present
(the topology is all described by firmware tables such as device-tree or
IORT), then we can safely use the DMA API for all legacy virtio devices.
Modern devices can advertise the prescense of an IOMMU using the
VIRTIO_F_IOMMU_PLATFORM feature flag.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: <stable@vger.kernel.org>
Fixes: 876945db ("arm64: Hook up IOMMU dma_ops")
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Acked-by: NMarc Zyngier <marc.zyngier@arm.com>

The device (not the driver) populates the used ring and includes the len
of how much data was written.
Signed-off-by: NFelipe Franciosi <felipe@nutanix.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

 # make C=2 CF="-D__CHECK_ENDIAN__" ./drivers/virtio/

drivers/virtio/virtio_ring.c:423:19: warning: incorrect type in assignment (different base types)
drivers/virtio/virtio_ring.c:423:19:    expected unsigned int [unsigned] [assigned] i
drivers/virtio/virtio_ring.c:423:19:    got restricted __virtio16 [usertype] next
drivers/virtio/virtio_ring.c:423:19: warning: incorrect type in assignment (different base types)
drivers/virtio/virtio_ring.c:423:19:    expected unsigned int [unsigned] [assigned] i
drivers/virtio/virtio_ring.c:423:19:    got restricted __virtio16 [usertype] next
drivers/virtio/virtio_ring.c:423:19: warning: incorrect type in assignment (different base types)
drivers/virtio/virtio_ring.c:423:19:    expected unsigned int [unsigned] [assigned] i
drivers/virtio/virtio_ring.c:423:19:    got restricted __virtio16 [usertype] next
drivers/virtio/virtio_ring.c:604:39: warning: incorrect type in initializer (different base types)
drivers/virtio/virtio_ring.c:604:39:    expected unsigned short [unsigned] [usertype] nextflag
drivers/virtio/virtio_ring.c:604:39:    got restricted __virtio16
drivers/virtio/virtio_ring.c:612:33: warning: restricted __virtio16 degrades to integer
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

This inline function is unused on configurations
where dma_map/unmap are empty macros.

Make the function inline to avoid gcc errors because
of an unused static function.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

According to the spec, if the VIRTIO_RING_F_EVENT_IDX feature bit is
negotiated the driver MUST set flags to 0. Not dirtying the available
ring in virtqueue_disable_cb also has a minor positive performance
impact, improving L1 dcache load missed by ~0.5% in vring_bench.

Writes to the used event field (vring_used_event) are still unconditional.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: <stable@vger.kernel.org> # f277ec42 virtio_ring: shadow available
Cc: <stable@vger.kernel.org>
Signed-off-by: NLadi Prosek <lprosek@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

We get 1 warning when building kernel with W=1:
drivers/virtio/virtio_ring.c:170:16: warning: no previous prototype for 'vring_dma_dev' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
so this patch marks this function with 'static'.
Signed-off-by: NBaoyou Xie <baoyou.xie@linaro.org>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

On error, virtqueue_add calls START_USE but not
END_USE. Thankfully that's normally empty anyway,
but might not be when debugging. Fix it up.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

When using the indirect buffers feature, 'desc' is allocated in
virtqueue_add() but isn't freed before leaving on a ring full error,
causing a memory leak.

For example, it seems rather clear that this can trigger
with virtio net if mergeable buffers are not used.

Cc: stable@vger.kernel.org
Signed-off-by: NWei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

The interaction between virtio and IOMMUs is messy.

On most systems with virtio, physical addresses match bus addresses,
and it doesn't particularly matter which one we use to program
the device.

On some systems, including Xen and any system with a physical device
that speaks virtio behind a physical IOMMU, we must program the IOMMU
for virtio DMA to work at all.

On other systems, including SPARC and PPC64, virtio-pci devices are
enumerated as though they are behind an IOMMU, but the virtio host
ignores the IOMMU, so we must either pretend that the IOMMU isn't
there or somehow map everything as the identity.

Add a feature bit to detect that quirk: VIRTIO_F_IOMMU_PLATFORM.

Any device with this feature bit set to 0 needs a quirk and has to be
passed physical addresses (as opposed to bus addresses) even though
the device is behind an IOMMU.

Note: it has to be a per-device quirk because for example, there could
be a mix of passed-through and virtual virtio devices. As another
example, some devices could be implemented by an out of process
hypervisor backend (in case of qemu vhost, or vhost-user) and so support
for an IOMMU needs to be coded up separately.

It would be cleanest to handle this in IOMMU core code, but that needs
per-device DMA ops. While we are waiting for that to be implemented, use
a work-around in virtio core.

Note: a "noiommu" feature is a quirk - add a wrapper to make
that clear.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

Smatch complains that we might not initialize "queue".  The issue is
callers like setup_vq() from virtio_pci_modern.c where "num" could be
something like 2 and "vring_align" is 64.  In that case, vring_size() is
less than PAGE_SIZE.  It won't happen in real life, but we're getting
the value of "num" from a register so it's not really possible to tell
what value it holds with static analysis.

Let's just silence the warning.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

Signed-off-by: NAndy Lutomirski <luto@kernel.org>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NDavid Vrabel <david.vrabel@citrix.com>
Reviewed-by: NWei Liu <wei.liu2@citrix.com>

This leaves vring_new_virtqueue alone for compatbility, but it
adds two new improved APIs:

vring_create_virtqueue: Creates a virtqueue backed by automatically
allocated coherent memory.  (Some day it this could be extended to
support non-coherent memory, too, if there ends up being a platform
on which it's worthwhile.)

__vring_new_virtqueue: Creates a virtqueue with a manually-specified
layout.  This should allow mic_virtio to work much more cleanly.
Signed-off-by: NAndy Lutomirski <luto@kernel.org>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

virtio_ring currently sends the device (usually a hypervisor)
physical addresses of its I/O buffers.  This is okay when DMA
addresses and physical addresses are the same thing, but this isn't
always the case.  For example, this never works on Xen guests, and
it is likely to fail if a physical "virtio" device ever ends up
behind an IOMMU or swiotlb.

The immediate use case for me is to enable virtio on Xen guests.
For that to work, we need vring to support DMA address translation
as well as a corresponding change to virtio_pci or to another
driver.
Signed-off-by: NAndy Lutomirski <luto@kernel.org>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

This is a kludge, but no one has come up with a a better idea yet.
We'll introduce DMA API support guarded by vring_use_dma_api().
Eventually we may be able to return true on more and more systems,
and hopefully we can get rid of vring_use_dma_api() entirely some
day.
Signed-off-by: NAndy Lutomirski <luto@kernel.org>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

We need a full barrier after writing out event index, using
virt_store_mb there seems better than open-coding.  As usual, we need a
wrapper to account for strong barriers.

It's tempting to use this in vhost as well, for that, we'll
need a variant of smp_store_mb that works on __user pointers.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Acked-by: NPeter Zijlstra (Intel) <peterz@infradead.org>

Improves cacheline transfer flow of available ring header.

Virtqueues are implemented as a pair of rings, one producer->consumer
avail ring and one consumer->producer used ring; preceding the
avail ring in memory are two contiguous u16 fields -- avail->flags
and avail->idx. A producer posts work by writing to avail->idx and
a consumer reads avail->idx.

The flags and idx fields only need to be written by a producer CPU
and only read by a consumer CPU; when the producer and consumer are
running on different CPUs and the virtio_ring code is structured to
only have source writes/sink reads, we can continuously transfer the
avail header cacheline between 'M' states between cores. This flow
optimizes core -> core bandwidth on certain CPUs.

(see: "Software Optimization Guide for AMD Family 15h Processors",
Section 11.6; similar language appears in the 10h guide and should
apply to CPUs w/ exclusive caches, using LLC as a transfer cache)

Unfortunately the existing virtio_ring code issued reads to the
avail->idx and read-modify-writes to avail->flags on the producer.

This change shadows the flags and index fields in producer memory;
the vring code now reads from the shadows and only ever writes to
avail->flags and avail->idx, allowing the cacheline to transfer
core -> core optimally.

In a concurrent version of vring_bench, the time required for
10,000,000 buffer checkout/returns was reduced by ~2% (average
across many runs) on an AMD Piledriver (15h) CPU:

(w/o shadowing):
 Performance counter stats for './vring_bench':
     5,451,082,016      L1-dcache-loads
     ...
       2.221477739 seconds time elapsed

(w/ shadowing):
 Performance counter stats for './vring_bench':
     5,405,701,361      L1-dcache-loads
     ...
       2.168405376 seconds time elapsed

The further away (in a NUMA sense) virtio producers and consumers are
from each other, the more we expect to benefit. Physical implementations
of virtio devices and implementations of virtio where the consumer polls
vring avail indexes (vhost) should also benefit.
Signed-off-by: NVenkatesh Srinivas <venkateshs@google.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

b92b1b89 ("virtio: force vring descriptors to be allocated from
lowmem") tried to exclude highmem pages for descriptors so it cleared
__GFP_HIGHMEM from a given gfp mask. The patch also cleared __GFP_HIGH
which doesn't make much sense for this fix because __GFP_HIGH only
controls access to memory reserves and it doesn't have any influence
on the zone selection. Some of the call paths use GFP_ATOMIC and
dropping __GFP_HIGH will reduce their changes for success because the
lack of access to memory reserves.
Signed-off-by: NMichal Hocko <mhocko@suse.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Acked-by: NWill Deacon <will.deacon@arm.com>
Reviewed-by: NMel Gorman <mgorman@techsingularity.net>

The virtqueue_add() calls START_USE() upon entry. The virtqueue_kick() is
called if vq->num_added == (1 << 16) - 1 before calling END_USE().
The virtqueue_kick_prepare() called via virtqueue_kick() calls START_USE()
upon entry, and will call panic() if DEBUG is enabled.
Move this virtqueue_kick() call to after END_USE() call.
Signed-off-by: NTetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Most of our code has
struct foo {
}

Fix one instances where ring is inconsistent.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Activate VIRTIO_F_VERSION_1 automatically unless legacy_only
is set.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

For virtio-1, we can theoretically have a more complex virtqueue
layout with avail and used buffers not on a contiguous memory area
with the descriptor table. For now, it's fine for a transport driver
to stay with the old layout: It needs, however, a way to access
the locations of the avail/used rings so it can register them with
the host.
Reviewed-by: NDavid Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

Use virtioXX_to_cpu and friends for access to
all multibyte structures in memory.

Note: this is intentionally mechanical.
A follow-up patch will split long lines etc.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NCornelia Huck <cornelia.huck@de.ibm.com>

It seemed like a good idea to use bitmap for features
in struct virtio_device, but it's actually a pain,
and seems to become even more painful when we get more
than 32 feature bits.  Just change it to a u32 for now.

Based on patch by Rusty.
Suggested-by: NDavid Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: NCornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NCornelia Huck <cornelia.huck@de.ibm.com>

virtqueue_add() populates the virtqueue descriptor table from the sgs
given.  If it uses an indirect descriptor table, then it puts a single
descriptor in the descriptor table pointing to the kmalloc'ed indirect
table where the sg is populated.

Previously vring_add_indirect() did the allocation and the simple
linear layout.  We replace that with alloc_indirect() which allocates
the indirect table then chains it like the normal descriptor table so
we can reuse the core logic.

This slows down pktgen by less than 1/2 a percent (which uses direct
descriptors), as well as vring_bench, but it's far neater.

vring_bench before:
	1061485790-1104800648(1.08254e+09+/-6.6e+06)ns
vring_bench after:
	1125610268-1183528965(1.14172e+09+/-8e+06)ns

pktgen before:
   787781-796334(793165+/-2.4e+03)pps 365-369(367.5+/-1.2)Mb/sec (365530384-369498976(3.68028e+08+/-1.1e+06)bps) errors: 0

pktgen after:
   779988-790404(786391+/-2.5e+03)pps 361-366(364.35+/-1.3)Mb/sec (361914432-366747456(3.64885e+08+/-1.2e+06)bps) errors: 0

Now, if we make force indirect descriptors by turning off any_header_sg
in virtio_net.c:

pktgen before:
  713773-721062(718374+/-2.1e+03)pps 331-334(332.95+/-0.92)Mb/sec (331190672-334572768(3.33325e+08+/-9.6e+05)bps) errors: 0
pktgen after:
  710542-719195(714898+/-2.4e+03)pps 329-333(331.15+/-1.1)Mb/sec (329691488-333706480(3.31713e+08+/-1.1e+06)bps) errors: 0
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

We used to have several callers which just used arrays.  They're
gone, so we can use sg_next() everywhere, simplifying the code.

On my laptop, this slowed down vring_bench by 15%:

vring_bench before:
	936153354-967745359(9.44739e+08+/-6.1e+06)ns
vring_bench after:
	1061485790-1104800648(1.08254e+09+/-6.6e+06)ns

However, a more realistic test using pktgen on a AMD FX(tm)-8320 saw
a few percent improvement:

pktgen before:
  767390-792966(785159+/-6.5e+03)pps 356-367(363.75+/-2.9)Mb/sec (356068960-367936224(3.64314e+08+/-3e+06)bps) errors: 0

pktgen after:
   787781-796334(793165+/-2.4e+03)pps 365-369(367.5+/-1.2)Mb/sec (365530384-369498976(3.68028e+08+/-1.1e+06)bps) errors: 0
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Good for post-apocalyptic scenarios, like S/390 hotplug.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Heinz points out that adding buffers to a broken virtqueue (which
should "never happen") still works.  Failing allows drivers to detect
and complain about broken devices.

Now drivers are robust, we can add this extra check.
Reported-by: NHeinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

In commit bb478d8b virtio_ring: plug kmemleak false positive,
kmemleak_ignore was introduced. This broke compilation of virtio_test:

  cc -g -O2 -Wall -I. -I ../../usr/include/ -Wno-pointer-sign
    -fno-strict-overflow -fno-strict-aliasing -fno-common -MMD
    -U_FORTIFY_SOURCE   -c -o virtio_ring.o ../../drivers/virtio/virtio_ring.c
  ../../drivers/virtio/virtio_ring.c: In function ‘vring_add_indirect’:
  ../../drivers/virtio/virtio_ring.c:177:2: warning: implicit declaration
  of function ‘kmemleak_ignore’ [-Wimplicit-function-declaration]
    kmemleak_ignore(desc);
    ^
  cc   virtio_test.o virtio_ring.o   -o virtio_test
  virtio_ring.o: In function `vring_add_indirect':
  tools/virtio/../../drivers/virtio/virtio_ring.c:177:
  undefined reference to `kmemleak_ignore'

Add a dummy header for tools/virtio, and add #incldue <linux/kmemleak.h>
to drivers/virtio/virtio_ring.c so it is picked up by the userspace
tools.
Signed-off-by: NJoel Stanley <joel@jms.id.au>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Correct if statement to check for bool returned by notify()
(introduced in 5b1bf7cb).
Signed-off-by: NHeinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Add new function virtqueue_is_broken(). Callers of virtqueue_get_buf()
should check for a broken queue.
Signed-off-by: NHeinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

virtqueue_{kick()/notify()} should exploit the new host notification API.
If the notify call returned with a negative value the host kick failed
(e.g. a kick triggered after a device was hot-unplugged). In this case
the virtqueue is set to 'broken' and false is returned, otherwise true.
Signed-off-by: NHeinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Currently a host kick error is silently ignored and not reflected in
the virtqueue of a particular virtio device.

Changing the notify API for guest->host notification seems to be one
prerequisite in order to be able to handle such errors in the context
where the kick is triggered.

This patch changes the notify API. The notify function must return a
bool return value. It returns false if the host notification failed.
Signed-off-by: NHeinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

unreferenced object 0xffff88003d467e20 (size 32):
  comm "softirq", pid 0, jiffies 4295197765 (age 6.364s)
  hex dump (first 32 bytes):
    28 19 bf 3d 00 00 00 00 0c 00 00 00 01 00 01 00  (..=............
    02 dc 51 3c 00 00 00 00 56 00 00 00 00 00 00 00  ..Q<....V.......
  backtrace:
    [<ffffffff8152db19>] kmemleak_alloc+0x59/0xc0
    [<ffffffff81102e93>] __kmalloc+0xf3/0x180
    [<ffffffff812db5d6>] vring_add_indirect+0x36/0x280
    [<ffffffff812dc59f>] virtqueue_add_outbuf+0xbf/0x4e0
    [<ffffffff813a8b30>] start_xmit+0x1a0/0x3b0
    [<ffffffff81445861>] dev_hard_start_xmit+0x2d1/0x4d0
    [<ffffffff81460052>] sch_direct_xmit+0xf2/0x1c0
    [<ffffffff81445c28>] dev_queue_xmit+0x1c8/0x460
    [<ffffffff814e3187>] ip6_finish_output2+0x1d7/0x470
    [<ffffffff814e34b0>] ip6_finish_output+0x90/0xb0
    [<ffffffff814e3507>] ip6_output+0x37/0xb0
    [<ffffffff815021eb>] igmp6_send+0x2db/0x470
    [<ffffffff81502645>] igmp6_timer_handler+0x95/0xa0
    [<ffffffff8104b57c>] call_timer_fn+0x2c/0x90
    [<ffffffff8104b7ba>] run_timer_softirq+0x1da/0x1f0
    [<ffffffff81045721>] __do_softirq+0xd1/0x1b0

Address gets embedded in a descriptor via virt_to_phys().  See detach_buf,
which frees it:

	if (vq->vring.desc[i].flags & VRING_DESC_F_INDIRECT)
		kfree(phys_to_virt(vq->vring.desc[i].addr));
Reported-by: NChristoph Paasch <christoph.paasch@uclouvain.be>
Fix-suggested-by: NChristoph Paasch <christoph.paasch@uclouvain.be>
Typing-done-by: NRusty Russell <rusty@rustcorp.com.au>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

This adds a way to check ring empty state after enable_cb outside any
locks. Will be used by virtio_net.

Note: there's room for more optimization: caller is likely to have a
memory barrier already, which means we might be able to get rid of a
barrier here.  Deferring this optimization until we do some
benchmarking.
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

All users changed to virtqueue_add_sg() or virtqueue_add_outbuf/inbuf.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

These are specialized versions of virtqueue_add_buf(), which cover
over 80% of cases and are far clearer.

In particular, the scatterlists passed to these functions don't have
to be clean (ie. we ignore end markers).
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

virtio_scsi can really use this, to avoid the current hack of copying
the whole sg array.  Some other things get slightly neater, too.

This causes a slowdown in virtqueue_add_buf(), which is implemented as
a wrapper.  This is addressed in the next patches.

for i in `seq 50`; do /usr/bin/time -f 'Wall time:%e' ./vringh_test --indirect --eventidx --parallel --fast-vringh; done 2>&1 | stats --trim-outliers:

Before:
	Using CPUS 0 and 3
	Guest: notified 0, pinged 39009-39063(39062)
	Host: notified 39009-39063(39062), pinged 0
	Wall time:1.700000-1.950000(1.723542)

After:
	Using CPUS 0 and 3
	Guest: notified 0, pinged 39062-39063(39063)
	Host: notified 39062-39063(39063), pinged 0
	Wall time:1.760000-2.220000(1.789167)
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Reviewed-by: NWanlong Gao <gaowanlong@cn.fujitsu.com>
Reviewed-by: NAsias He <asias@redhat.com>

The host side of ring needs this logic too.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Now noone relies on this behavior, we simplify virtqueue_add_buf() so it
return 0 or -errno.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>