Jeffrey added Multi-MSI support to the pci-hyperv driver by the 4 patches:
08e61e86 ("PCI: hv: Fix multi-MSI to allow more than one MSI vector")
455880df ("PCI: hv: Fix hv_arch_irq_unmask() for multi-MSI")
b4b77778 ("PCI: hv: Reuse existing IRTE allocation in compose_msi_msg()")
a2bad844 ("PCI: hv: Fix interrupt mapping for multi-MSI")

It turns out that the third patch (b4b77778) causes a performance
regression because all the interrupts now happen on 1 physical CPU (or two
pCPUs, if one pCPU doesn't have enough vectors). When a guest has many PCI
devices, it may suffer from soft lockups if the workload is heavy, e.g.,
see https://lwn.net/ml/linux-kernel/20220804025104.15673-1-decui@microsoft.com/

Commit b4b77778 itself is good. The real issue is that the hypercall in
hv_irq_unmask() -> hv_arch_irq_unmask() ->
hv_do_hypercall(HVCALL_RETARGET_INTERRUPT...) only changes the target
virtual CPU rather than physical CPU; with b4b77778, the pCPU is
determined only once in hv_compose_msi_msg() where only vCPU0 is specified;
consequently the hypervisor only uses 1 target pCPU for all the interrupts.

Note: before b4b77778, the pCPU is determined twice, and when the pCPU
is determined the second time, the vCPU in the effective affinity mask is
used (i.e., it isn't always vCPU0), so the hypervisor chooses different
pCPU for each interrupt.

The hypercall will be fixed in future to update the pCPU as well, but
that will take quite a while, so let's restore the old behavior in
hv_compose_msi_msg(), i.e., don't reuse the existing IRTE allocation for
single-MSI and MSI-X; for multi-MSI, we choose the vCPU in a round-robin
manner for each PCI device, so the interrupts of different devices can
happen on different pCPUs, though the interrupts of each device happen on
some single pCPU.

The hypercall fix may not be backported to all old versions of Hyper-V, so
we want to have this guest side change forever (or at least till we're sure
the old affected versions of Hyper-V are no longer supported).

Fixes: b4b77778 ("PCI: hv: Reuse existing IRTE allocation in compose_msi_msg()")
Co-developed-by: NJeffrey Hugo <quic_jhugo@quicinc.com>
Signed-off-by: NJeffrey Hugo <quic_jhugo@quicinc.com>
Co-developed-by: NCarl Vanderlip <quic_carlv@quicinc.com>
Signed-off-by: NCarl Vanderlip <quic_carlv@quicinc.com>
Signed-off-by: NDexuan Cui <decui@microsoft.com>
Reviewed-by: NMichael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20221104222953.11356-1-decui@microsoft.comSigned-off-by: NWei Liu <wei.liu@kernel.org>