Re: [Qemu-devel] Virtual IOMMU is working for Windows VM?

[Jintack Lim <jintack@cs.columbia.edu>]

On Tue, Oct 23, 2018 at 9:25 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Mon, Oct 22, 2018 at 10:43:32AM -0400, Jintack Lim wrote:
> > On Mon, Oct 22, 2018 at 5:27 AM Peter Xu <peterx@redhat.com> wrote:
> > >
> > > On Mon, Oct 22, 2018 at 12:22:02AM -0400, Jintack Lim wrote:
> > > > Hi,
> > > >
> > > > I wonder if vIOMMU is working for Windows VM?
> > > >
> > > > I tried it with v2.11.0, but it didn't seem to work. I assume that seaBIOS
> > > > sets IOMMU on by default as is the case when I launched a Linux VM. But I
> > > > might be missing something. Can somebody shed some light on it?
> > >
> > > Hi, Jintack,
> > >
> >
> > Thanks Peter,
> >
> > > I think at least the latest QEMU should work for Windows, but I don't
> > > really run Windows that frequently.
> > >
> > > What is the error you've encountered?  Have you tried the latest QEMU,
> > > or switching Windows versions to try?
> >
> > I ran Windows commands in Windows Powershell like below. Well, I guess
> > this is not the best way to check IOMMU presence, but couldn't find a
> > better way to do it.
> >
> > $ (Get-VMHost).IovSupport
> > false
> > $ (Get-VMHost).IovSupportReasons
> > The chipset on the system does not do DMA remapping, ...
> >
> > I just tried QEMU v3.0.0, but I see the same symptom. I'm using
> > Windows server 2016.  Unfortunately, trying another Windows version
> > would be hard for me at this point.
> >
> > I just wonder if there's way to check if Vt-d is on in SeaBIOS?
>
> I'm not sure whether SeaBIOS would enable VT-d or has any kind of
> support of it at all even if the translation unit is provided.
>

All right. I then assume the BIOS doesn't disable it as is the case
for Linux guest.

> >
> > >
> > > What I can remember about Windows is that Ladi had fixed a bug for
> > > windows-only (8991c460be, "intel_iommu: relax iq tail check on
> > > VTD_GCMD_QIE enable", 2017-07-03) but it should be even in 2.10 so I
> > > guess it's not the problem you've encountered.
> >
> > I'm CCing Ladi, just in case he has some idea :)
>
> Good idea, though I'm afraid the RH email could be stall though. :)

Ah.. :)

>
> I can try to install one Windows Server 2016 some day but I cannot
> really guarantee.  Feel free to try to debug it on your own :).
> Basically I would consider to enable the IOMMU traces in intel_iommu.c
> just like what you have done before when with the vfio-pci devices and
> check out the log.  Normally we should see plenty of MMIOs to setup
> the device and hopefully that would provide hint on what's wrong there.

Thanks. I will. Now that Linux guest can recognize vIOMMU well, hope I
can spot where things go wrong with Window guest hopefully!

Best,
Jintack

>
> Regards,
>
> --
> Peter Xu
>