[Qemu-devel] MSI interrupt support with vioscsi.c miniport driver

[Nicholas A. Bellinger]

Hi Yan,

So recently I've been doing some KVM guest performance comparisons
between the scsi-mq prototype using virtio-scsi + vhost-scsi, and
Windows Server 2012 with vioscsi.sys (virtio-win-0.1-74.iso) +
vhost-scsi using PCIe flash backend devices.

I've noticed that small block random performance for the MSFT guest is
at around ~80K IOPs with multiple vioscsi LUNs + adapters, which ends up
being well below what the Linux guest with scsi-mq + virtio-scsi is
capable of (~500K).

After searching through the various vioscsi registry settings, it
appears that MSIEnabled is being explicitly disabled (0x00000000), that
is different from what vioscsi.inx is currently defining:

[pnpsafe_pci_addreg_msix]
HKR, "Interrupt Management",, 0x00000010
HKR, "Interrupt Management\MessageSignaledInterruptProperties",, 0x00000010
HKR, "Interrupt Management\MessageSignaledInterruptProperties", MSISupported, 
0x00010001, 0
HKR, "Interrupt Management\MessageSignaledInterruptProperties", 
MessageNumberLimit, 0x00010001, 4

Looking deeper at vioscsi.c code, I've noticed that MSI_SUPPORTED=0 is
explicitly disabled at build time in SOURCES + vioscsi.vcxproj, as well
as VioScsiFindAdapter() code always ends setting msix_enabled = FALSE
here, regardless of MSI_SUPPORTED:

  
https://github.com/YanVugenfirer/kvm-guest-drivers-windows/blob/master/vioscsi/vioscsi.c#L340

Also looking at virtio_stor.c for the raw block driver, MSI_SUPPORTED=1
appears to be the default setting for the driver included in the offical
virtio-win iso builds, right..?

Sooo, I'd like to try enabling MSI_SUPPORTED=1 in a test vioscsi.sys
build of my own, but before going down the WDK development rabbit whole,
I'd like to better understand why you've explicitly disabled this logic
within vioscsi.c code to start..?

Is there anything that needs to be addressed / carried over from
virtio_stor.c in order to get MSI_SUPPORTED=1 to work with vioscsi.c
miniport code..?

TIA!

--nab