Re: [Qemu-devel] using MSIX for virtio-blk

[Stefan Hajnoczi]

On Tue, Jul 07, 2015 at 10:50:59PM +0530, Naredula Janardhana Reddy wrote:
>  How to enable MSIX for virtio-blk, this is to avoid extra use of
> VIRTIO_PCI_ISR port reads.
> This is for qemu 2.3.0 with virtio-blk-pci with x-data-plane is on.

I tried the following with QEMU 2.3.0:

  qemu-system-x86_64 -enable-kvm -m 1024 \
                     -drive 
if=none,id=drive0,file=test.img,format=raw,aio=native,cache=none \
                     -object iothread,id=iothread0 \
                     -device virtio-blk-pci,drive=drive0,iothread=iothread0

Inside the guest I checked /proc/interrupts and found that the
virtio-pci/virtio-blk driver is using MSI.

I also checked /proc/ioports and found the virtio-pci registers at
0xc040.

Then I used "perf record -a -e kvm:kvm_pio" on the host to record all
PIO accesses.

Inside the guest I ran "dd if=/dev/vda of=/dev/null iflag=direct bs=4k"
for about 30 seconds.  Afterwards I ran "perf script" on the host and
subtracted the 0xc040 base address from the PIO accesses to identify the
virtio-pci registers.

I didn't see any accesses to the ISR register, only writes to the
QUEUE_NOTIFY register.

Either your guest operating system is not enabling MSI or you are using
QEMU command-line options that prevent MSI.

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