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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