Also removes a long-unused #define and an extraneous semicolon.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

We used to notify the Host every time we updated a device's status.  However,
it only really needs to know when we're resetting the device, or failed to
initialize it, or when we've finished our feature negotiation.

In particular, we used to wait for VIRTIO_CONFIG_S_DRIVER_OK in the
status byte before starting the device service threads.  But this
corresponds to the successful finish of device initialization, which
might (like virtio_blk's partition scanning) use the device.  So we
had a hack, if they used the device before we expected we started the
threads anyway.

Now we hook into the finalize_features hook in the Guest: at that
point we tell the Launcher that it can rely on the features we have
acked.  On the Launcher side, we look at the status at that point, and
start servicing the device.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Do not exit on some non-fatal errors:

- writev() fails in net_output(). The result is a lost packet or packets.
- writev() fails in console_output(). The result is partially lost console
output.
- readv() fails in net_input(). The result is a lost packet or packets.

Rather than bringing the guest down, this patch ignores e.g. an allocation
failure on the host side. Example:

lguest: page allocation failure. order:4, mode:0x4d0
Pid: 4045, comm: lguest Tainted: G        W   2.6.36 #1
Call Trace:
 [<c138d614>] ? printk+0x18/0x1c
 [<c106a4e2>] __alloc_pages_nodemask+0x4d2/0x570
 [<c1087954>] cache_alloc_refill+0x2a4/0x4d0
 [<c1305149>] ? __netif_receive_skb+0x189/0x270
 [<c1087c5a>] __kmalloc+0xda/0xf0
 [<c12fffa5>] __alloc_skb+0x55/0x100
 [<c1305519>] ? net_rx_action+0x79/0x100
 [<c12fafed>] sock_alloc_send_pskb+0x18d/0x280
 [<c11fda25>] ? _copy_from_user+0x35/0x130
 [<c13010b6>] ? memcpy_fromiovecend+0x56/0x80
 [<c12a74dc>] tun_chr_aio_write+0x1cc/0x500
 [<c108a125>] do_sync_readv_writev+0x95/0xd0
 [<c11fda25>] ? _copy_from_user+0x35/0x130
 [<c1089fa8>] ? rw_copy_check_uvector+0x58/0x100
 [<c108a7bc>] do_readv_writev+0x9c/0x1d0
 [<c12a7310>] ? tun_chr_aio_write+0x0/0x500
 [<c108a93a>] vfs_writev+0x4a/0x60
 [<c108aa21>] sys_writev+0x41/0x80
 [<c138f061>] syscall_call+0x7/0xb
Mem-Info:
DMA per-cpu:
CPU    0: hi:    0, btch:   1 usd:   0
Normal per-cpu:
CPU    0: hi:  186, btch:  31 usd:   0
HighMem per-cpu:
CPU    0: hi:  186, btch:  31 usd:   0
active_anon:134651 inactive_anon:50543 isolated_anon:0
 active_file:96881 inactive_file:132007 isolated_file:0
 unevictable:0 dirty:3 writeback:0 unstable:0
 free:91374 slab_reclaimable:6300 slab_unreclaimable:2802
 mapped:2281 shmem:9 pagetables:330 bounce:0
DMA free:3524kB min:64kB low:80kB high:96kB active_anon:0kB inactive_anon:8kB active_file:8760kB inactive_file:2760kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:15868kB mlocked:0kB dirty:0kB writeback:0kB mapped:16kB shmem:0kB slab_reclaimable:88kB slab_unreclaimable:148kB kernel_stack:40kB pagetables:0kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? no
lowmem_reserve[]: 0 865 2016 2016
Normal free:150100kB min:3728kB low:4660kB high:5592kB active_anon:6224kB inactive_anon:15772kB active_file:324084kB inactive_file:325944kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:885944kB mlocked:0kB dirty:12kB writeback:0kB mapped:1520kB shmem:0kB slab_reclaimable:25112kB slab_unreclaimable:11060kB kernel_stack:1888kB pagetables:1320kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? no
lowmem_reserve[]: 0 0 9207 9207
HighMem free:211872kB min:512kB low:1752kB high:2992kB active_anon:532380kB inactive_anon:186392kB active_file:54680kB inactive_file:199324kB unevictable:0kB isolated(anon):0kB isolated(file):0kB present:1178504kB mlocked:0kB dirty:0kB writeback:0kB mapped:7588kB shmem:36kB slab_reclaimable:0kB slab_unreclaimable:0kB kernel_stack:0kB pagetables:0kB unstable:0kB bounce:0kB writeback_tmp:0kB pages_scanned:0 all_unreclaimable? no
lowmem_reserve[]: 0 0 0 0
DMA: 3*4kB 65*8kB 35*16kB 18*32kB 11*64kB 9*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 3524kB
Normal: 35981*4kB 344*8kB 158*16kB 28*32kB 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 150100kB
HighMem: 5732*4kB 5462*8kB 2826*16kB 1598*32kB 84*64kB 10*128kB 7*256kB 1*512kB 1*1024kB 1*2048kB 9*4096kB = 211872kB
231237 total pagecache pages
2340 pages in swap cache
Swap cache stats: add 160060, delete 157720, find 189017/194106
Free swap  = 4179840kB
Total swap = 4194300kB
524271 pages RAM
296946 pages HighMem
5668 pages reserved
867664 pages shared
82155 pages non-shared
Signed-off-by: NSakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

No virtio device does this any more, so no need to clutter lguest with it.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

ed16648e "Move kvm, uml, and lguest
subdirectories" broke the lguest example launcher.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

  cd Documentation
  mkdir virtual
  git mv kvm uml lguest virtual
Signed-off-by: NRob Landley <rlandley@parallels.com>
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>

PROT_EXEC seems to be completely unnecessary (as the lguest binary
never executes there), and will allow it to work with SELinux (and
more importantly, PaX :-) as they can/do forbid writable and
executable mappings.

Also, map PROT_NONE guard pages at start and end of guest memory for extra
paranoia.

I changed the length check to addr + size > guest_limit because >= is wrong
(addr of 0, size of getpagesize() with a guest_limit of getpagesize() would
false positive).
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

I've attached a patch which implements dropping to privileges
and chrooting to a directory.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

VIRTIO_F_BARRIER is deprecated.  Replace it with VIRTIO_F_FLUSH
support.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <jaxboe@fusionio.com>

These days the headers we use are in glibc.  If those are too old, you can
add the -I lines to get the kernel headers.

In file included from ../../include/linux/if_tun.h:19,
                 from lguest.c:33:
../../include/linux/types.h:13:2: warning: #warning "Attempt to use kernel headers from user space, see http://kernelnewbies.org/KernelHeaders"
lguest.c: In function ‘setup_tun_net’:
lguest.c:1456: warning: dereferencing pointer ‘sin’ does break strict-aliasing rules
lguest.c:1457: warning: dereferencing pointer ‘sin’ does break strict-aliasing rules
lguest.c:1450: note: initialized from here
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Fix obvious cases of "it's" being used when "its" was meant.
Signed-off-by: NFrancis Galiegue <fgaliegue@gmail.com>
Acked-by: NRandy Dunlap <rdunlap@xenotime.net>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

Two years ago 5bbf89fc removed the horrible bzImage unpacking code.
Now it's time to remove the unneeded zlib.h include, too.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

That is "success", "unknown", "through", "performance", "[re|un]mapping"
, "access", "default", "reasonable", "[con]currently", "temperature"
, "channel", "[un]used", "application", "example","hierarchy", "therefore"
, "[over|under]flow", "contiguous", "threshold", "enough" and others.
Signed-off-by: NAndré Goddard Rosa <andre.goddard@gmail.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

Rusty,

commit 3ca4f5ca
    virtio: add virtio IDs file
moved all device IDs into a single file. While the change itself is
a very good one, it can break userspace applications. For example
if a userspace tool wanted to get the ID of virtio_net it used to
include virtio_net.h. This does no longer work, since virtio_net.h
does not include virtio_ids.h.
This patch moves all "#include <linux/virtio_ids.h>" from the C
files into the header files, making the header files compatible with
the old ones.

In addition, this patch exports virtio_ids.h to userspace.

CC: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: NChristian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

VIRTIO_F_NOTIFY_ON_EMPTY indicates to the Guest that we will hit them with
an interrupt every time the xmit queue is emptied.

Because it results in lots of tx interrupts, modern Guests probably don't
want it, so let's only force it when they accept the option.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Virtio IDs are spread all over the tree which makes assigning new IDs
bothersome. Putting them together should make the process less error-prone.
Signed-off-by: NFernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

I've been doing this for years, and akpm picked me up on it about 12
months ago.  lguest partly serves as example code, so let's do it Right.

Also, remove two unused fields in struct vblk_info in the example launcher.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@redhat.com>

Every so often, after code shuffles, I need to go through and unbitrot
the Lguest Journey (see drivers/lguest/README).  Since we now use RCU in
a simple form in one place I took the opportunity to expand that explanation.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>

I don't really notice it (except to begrudge the extra vertical
space), but Ingo does.  And he pointed out that one excuse of lguest
is as a teaching tool, it should set a good example.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@redhat.com>

1d589bb1 "Add serial number support
for virtio_blk, V4a" extended 'struct virtio_blk_config' to 536 bytes.
Lguest and S/390 both use an 8 bit value for the feature length, and
this change broke them (if the code is naive).
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Cc: John Cooper <john.cooper@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>

Support the VIRTIO_RING_F_INDIRECT_DESC feature.

This is a simple matter of changing the descriptor walking
code to operate on a struct vring_desc* and supplying it
with an indirect table if detected.
Signed-off-by: NMark McLoughlin <markmc@redhat.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

The Guest only really needs to tell us about activity when we're going
to listen to the eventfd: normally, we don't want to know.

So if there are no available buffers, turn on notifications, re-check,
then wait for the Guest to notify us via the eventfd, then turn
notifications off again.

There's enough else going on that the differences are in the noise.

Before:				Secs	RxKicks	TxKicks
 1G TCP Guest->Host:		3.94	  4686	  32815
 1M normal pings:		104	142862	1000010
 1M 1k pings (-l 120):		57	142026	1000007

After:
 1G TCP Guest->Host:		3.76	  4691	  32811
 1M normal pings:		111	142859	 997467
 1M 1k pings (-l 120):		55	 19648	 501549
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Rather than triggering an interrupt every time, we only trigger an
interrupt when there are no more incoming packets (or the recv queue
is full).

However, the overhead of doing the select to figure this out is
measurable: 1M pings goes from 98 to 104 seconds, and 1G Guest->Host
TCP goes from 3.69 to 3.94 seconds.  It's close to the noise though.

I tested various timeouts, including reducing it as the number of
pending packets increased, timing a 1 gigabyte TCP send from Guest ->
Host and Host -> Guest (GSO disabled, to increase packet rate).

// time tcpblast -o -s 65536 -c 16k 192.168.2.1:9999 > /dev/null

Timeout		Guest->Host	Pkts/irq	Host->Guest	Pkts/irq
Before		11.3s		1.0		6.3s		1.0
0		11.7s		1.0		6.6s		23.5
1		17.1s		8.8		8.6s		26.0
1/pending	13.4s		1.9		6.6s		23.8
2/pending	13.6s		2.8		6.6s		24.1
5/pending	14.1s		5.0		6.6s		24.4
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

If we track how many buffers we've used, we can tell whether we really
need to interrupt the Guest.  This happens as a side effect of
spurious notifications.

Spurious notifications happen because it can take a while before the
Host thread wakes up and sets the VRING_USED_F_NO_NOTIFY flag, and
meanwhile the Guest can more notifications.

A real fix would be to use wake counts, rather than a suppression
flag, but the practical difference is generally in the noise: the
interrupt is usually coalesced into a pending one anyway so we just
save a system call which isn't clearly measurable.

				Secs	Spurious IRQS
1G TCP Guest->Host:		3.93	58
1M normal pings:		100	72
1M 1k pings (-l 120):		57	492904
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Rather than sending an interrupt on every buffer, we only send an interrupt
when we're about to wait for the Guest to send us a new one.  The console
input and network input still send interrupts manually, but the block device,
network and console output queues can simply rely on this logic to send
interrupts to the Guest at the right time.

The patch is cluttered by moving trigger_irq() higher in the code.

In practice, two factors make this optimization less interesting:
(1) we often only get one input at a time, even for networking,
(2) triggering an interrupt rapidly tends to get coalesced anyway.

Before:				Secs	RxIRQS	TxIRQs
 1G TCP Guest->Host:		3.72	32784	32771
 1M normal pings:		99	1000004	995541
 100,000 1k pings (-l 120):	5	49510	49058

After:
 1G TCP Guest->Host:		3.69	32809	32769
 1M normal pings:		99	1000004	996196
 100,000 1k pings (-l 120):	5	52435	52361

(Note the interrupt count on 100k pings goes *up*: see next patch).
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Currently lguest has three threads: the main Launcher thread, a Waker
thread, and a thread for the block device (because synchronous block
was simply too painful to bear).

The Waker selects() on all the input file descriptors (eg. stdin, net
devices, pipe to the block thread) and when one becomes readable it calls
into the kernel to kick the Launcher thread out into userspace, which
repeats the poll, services the device(s), and then tells the kernel to
release the Waker before re-entering the kernel to run the Guest.

Also, to make a slightly-decent network transmit routine, the Launcher
would suppress further network interrupts while it set a timer: that
signal handler would write to a pipe, which would rouse the Waker
which would prod the Launcher out of the kernel to check the network
device again.

Now we can convert all our virtqueues to separate threads: each one has
a separate eventfd for when the Guest pokes the device, and can trigger
interrupts in the Guest directly.

The linecount shows how much this simplifies, but to really bring it
home, here's an strace analysis of single Guest->Host ping before:

* Guest sends packet, notifies xmit vq, return control to Launcher
* Launcher clears notification flag on xmit ring
* Launcher writes packet to TUN device
	writev(4, [{"\0\0\0\0\0\0\0\0\0\0", 10}, {"\366\r\224`\2058\272m\224vf\274\10\0E\0\0T\0\0@\0@\1\265"..., 98}], 2) = 108
* Launcher sets up interrupt for Guest (xmit ring is empty)
	write(10, "\2\0\0\0\3\0\0\0", 8) = 0
* Launcher sets up timer for interrupt mitigation
	setitimer(ITIMER_REAL, {it_interval={0, 0}, it_value={0, 505}}, NULL) = 0
* Launcher re-runs guest
	pread64(10, 0xbfa5f4d4, 4, 0) ...
* Waker notices reply packet in tun device (it was in select)
	select(12, [0 3 4 6 11], NULL, NULL, NULL) = 1 (in [4])
* Waker kicks Launcher out of guest:
	pwrite64(10, "\3\0\0\0\1\0\0\0", 8, 0) = 0
* Launcher returns from running guest:
	... = -1 EAGAIN (Resource temporarily unavailable)
* Launcher looks at input fds:
	select(7, [0 3 4 6], NULL, NULL, {0, 0}) = 1 (in [4], left {0, 0})
* Launcher reads pong from tun device:
	readv(4, [{"\0\0\0\0\0\0\0\0\0\0", 10}, {"\272m\224vf\274\366\r\224`\2058\10\0E\0\0T\364\26\0\0@"..., 1518}], 2) = 108
* Launcher injects guest notification:
	write(10, "\2\0\0\0\2\0\0\0", 8) = 0
* Launcher rechecks fds:
	select(7, [0 3 4 6], NULL, NULL, {0, 0}) = 0 (Timeout)
* Launcher clears Waker:
	pwrite64(10, "\3\0\0\0\0\0\0\0", 8, 0) = 0
* Launcher reruns Guest:
	pread64(10, 0xbfa5f4d4, 4, 0) = ? ERESTARTSYS (To be restarted)
* Signal comes in, uses pipe to wake up Launcher:
	--- SIGALRM (Alarm clock) @ 0 (0) ---
	write(8, "\0", 1)       = 1
	sigreturn()             = ? (mask now [])
* Waker sees write on pipe:
	select(12, [0 3 4 6 11], NULL, NULL, NULL) = 1 (in [6])
* Waker kicks Launcher out of Guest:
	pwrite64(10, "\3\0\0\0\1\0\0\0", 8, 0) = 0
* Launcher exits from kernel:
	pread64(10, 0xbfa5f4d4, 4, 0) = -1 EAGAIN (Resource temporarily unavailable)
* Launcher looks to see what fd woke it:
	select(7, [0 3 4 6], NULL, NULL, {0, 0}) = 1 (in [6], left {0, 0})
* Launcher reads timeout fd, sets notification flag on xmit ring
	read(6, "\0", 32)       = 1
* Launcher rechecks fds:
	select(7, [0 3 4 6], NULL, NULL, {0, 0}) = 0 (Timeout)
* Launcher clears Waker:
	pwrite64(10, "\3\0\0\0\0\0\0\0", 8, 0) = 0
* Launcher resumes Guest:
	pread64(10, "\0p\0\4", 4, 0) ....

strace analysis of single Guest->Host ping after:

* Guest sends packet, notifies xmit vq, creates event on eventfd.
* Network xmit thread wakes from read on eventfd:
	read(7, "\1\0\0\0\0\0\0\0", 8)          = 8
* Network xmit thread writes packet to TUN device
	writev(4, [{"\0\0\0\0\0\0\0\0\0\0", 10}, {"J\217\232FI\37j\27\375\276\0\304\10\0E\0\0T\0\0@\0@\1\265"..., 98}], 2) = 108
* Network recv thread wakes up from read on tunfd:
	readv(4, [{"\0\0\0\0\0\0\0\0\0\0", 10}, {"j\27\375\276\0\304J\217\232FI\37\10\0E\0\0TiO\0\0@\1\214"..., 1518}], 2) = 108
* Network recv thread sets up interrupt for the Guest
	write(6, "\2\0\0\0\2\0\0\0", 8) = 0
* Network recv thread goes back to reading tunfd
	13:39:42.460285 readv(4,  <unfinished ...>
* Network xmit thread sets up interrupt for Guest (xmit ring is empty)
	write(6, "\2\0\0\0\3\0\0\0", 8) = 0
* Network xmit thread goes back to reading from eventfd
	read(7, <unfinished ...>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

I've never seen it here, but I can't find anywhere that says writev
will write everything.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

The "len" field in the used ring for virtio indicates the number of
bytes *written* to the buffer.  This means the guest doesn't have to
zero the buffers in advance as it always knows the used length.

Erroneously, the console and network example code puts the length
*read* into that field.  The guest ignores it, but it's wrong.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

20887611 (lguest: notify on empty) introduced
lguest support for the VIRTIO_F_NOTIFY_ON_EMPTY flag, but in fact it turned on
interrupts all the time.

Because we always process one buffer at a time, the inflight count is always 0
when call trigger_irq and so we always ignore VRING_AVAIL_F_NO_INTERRUPT from
the Guest.

It should be looking to see if there are more buffers in the Guest's queue:
if it's empty, then we force an interrupt.

This makes little difference, since we usually have an empty queue; but
that's the subject of another patch.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Since the Launcher process runs the Guest, it doesn't have to be very
serious about its barriers: the Guest isn't running while we are (Guest
is UP).

Before we change to use threads to service devices, we need to fix this.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

We hand the /dev/lguest fd everywhere; it's far neater to just make it
a global (it already is, in fact, hidden in the waker_fds struct).
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

We can't trust the values in the device descriptor table once the
guest has booted, so keep local copies.  They could set them to
strange values then cause us to segv (they're 8 bit values, so they
can't make our pointers go too wild).

This becomes more important with the following patches which read them.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Impact: barrier correctness in example launcher

I doubt either lguest user will complain about performance.
Reported-by: NChristoph Hellwig <hch@infradead.org>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

This patch moves the initial guest page table creation code to the host,
so the launcher keeps working with PAE enabled configs.
Signed-off-by: NMatias Zabaljauregui <zabaljauregui@gmail.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

This doesn't really matter, since lguest is i386 only at the moment,
but we could actually choose a different value.  (lguest doesn't have
a guarenteed ABI).
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

The Documentation/i386 and Documentation/x86_64 directories and their
contents have been moved into Documentation/x86. Fix references to
those files accordingly.
Signed-off-by: NUwe Hermann <uwe@hermann-uwe.de>
Signed-off-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

This shows up when trying to bridge:
	tap0: received packet with  own address as source address

As Max Krasnyansky points out, there's no reason to give the guest the
same mac address as the TUN device.
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
Cc: Max Krasnyansky <maxk@qualcomm.com>

lguest uses a Waker process to break it out of the kernel (ie.
actually running the guest) when file descriptor needs attention.

Changing this from a process to a thread somewhat simplifies things:
it can directly access the fd_set of things to watch.  More
importantly, it means that the Waker can see Guest memory correctly,
so /dev/vring file descriptors will work as anticipated (the
alternative is to actually mmap MAP_SHARED, but you can't do that with
/dev/zero).
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>