Re: [Qemu-devel] MSI-X doesn't work when running Windows as guest

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Sep 12, 2013 at 04:45:01PM -0300, Eduardo Habkost wrote:
> On Thu, Sep 12, 2013 at 11:42:17AM +0300, Michael S. Tsirkin wrote:
> > On Thu, Sep 12, 2013 at 11:23:46AM +0300, Gal Hammer wrote:
> > > Hi,
> > > 
> > > I've notice that the virtio-serial Windows' driver doesn't use MSI-X
> > > vectors when running using upstream qemu or
> > > qemu-kvm-1.2.2-13.fc18.x86_64. The same VM works with MSI-X when
> > > using qemu-kvm-0.12.1.2-2.355.el6.x86_64.
> > > 
> > > From what I saw, Windows is trying to enable MSI-X by writing a 2
> > > bytes value to device's PCI-config address 66h.
> > > 
> > > So when everything works well the flow goes like this:
> > > 
> > > pci_default_write_config value: 8000 len: 2
> > > pci_default_write_config value: 1 len: 2
> > > msix_enabled 0 (67)
> > > pci_default_write_config value: e107 len: 2
> > > pci_default_write_config value: 1 len: 2
> > > msix_enabled 0 (67)
> > > pci_default_write_config value: 8001 len: 2
> > > msix_enabled 1 (67)
> > > 
> > > But on upstream it goes:
> > > 
> > > pci_default_write_config addr: 66 value: 8000 size: 2
> > > pci_default_write_config addr: 66 value: 1 size: 2
> > > msix_enabled 0 (67)
> > > pci_default_write_config addr: 66 value: e307 size: 2 (NOTE: Value
> > > is diffrent!).
> > > pci_default_write_config addr: 66 value: 1 size: 2
> > > msix_enabled 0 (67)
> > > 
> > > (NOTE: Missing the write of 8001).
> > > 
> > > My qemu's command line:
> > > 
> > > ---< snip >---
> > > 
> > > /usr/bin/qemu-kvm -m 1G -smp 2 -enable-kvm -usb -device usb-tablet \
> > >         -device
> > > ide-drive,drive=drive-virtio0-0-0,id=virtio0-0-0,bootindex=1 \
> > >         -drive file=win7_32_viorng.qcow2,if=none,id=drive-virtio0-0-0,format=qcow2,werror=stop,rerror=stop,cache=none
> > > \
> > >         -monitor stdio \
> > >         -vga qxl -spice id=on,disable-ticketing,port=5903 \
> > >         -device virtio-serial-pci,id=virtio-serial0,vectors=2 \
> > >         -chardev spicevmc,id=spicechannel0,name=vdagent
> > > 
> > > ---< snip >---
> > > 
> > > Thanks,
> > > 
> > >     Gal.
> > 
> > 
> > So it's a known change from qemu-kvm to qemu.
> > With qemu-kvm the default cpu was kvm64.
> > With qemu the default cpu is qemu64 even if you use -enable-kvm.
> > 
> > Not an issue for libvirt as that specifies -cpu,
> > but will be an issue for command-line users.
> > 
> > Maybe we should change the default for new machine types and when
> > -enable-kvm is specified?
> 
> What about simply making qemu64 as good as kvm64 (on newer
> machine-types)?

This will likely mean extending tcg to emulate more CPU
features. Do you want to spend cycles on this?

> What exactly is missing on qemu64 that causes the above
> problem?

I remember windows checks that cpu is modern enough
to enable msi-x.
Dont' remember the exact details.

> -- 
> Eduardo