Re: [PATCH] KVM: x86: Clear CR3[63:32] on SMM entry from long mode

Re: [PATCH] KVM: x86: Clear CR3[63:32] on SMM entry from long mode

[Qiliang Yuan]

+Cc Hyper-V folks and linux-hyperv, since the failure is triggered by
Hyper-V's handling of VMRUN for a nested (L1) hypervisor, i.e. KVM
running on WSL2.

For the Hyper-V side: the VMRUN is rejected when the guest state has
CR0.PG=0, EFER.LMA=0 and CR3[63:32] != 0, a combination that bare-metal
SVM does not reject.  Is that check intentional?  If it is not, a
Hyper-V fix would help older KVM versions as well; either way the patch
avoids creating that state in the first place.

The full patch is here:
https://lore.kernel.org/r/20260928-kvm-smm-cr3-upper-bits-v1-1-138f52dd531e@gmail.com

Thanks,
Qiliang

Re: [PATCH] KVM: x86: Clear CR3[63:32] on SMM entry from long mode

[Wei Liu]

Hi Qiliang

On Mon, Sep 28, 2026 at 03:25:34PM +0800, Qiliang Yuan wrote:
> +Cc Hyper-V folks and linux-hyperv, since the failure is triggered by
> Hyper-V's handling of VMRUN for a nested (L1) hypervisor, i.e. KVM
> running on WSL2.
> 
> For the Hyper-V side: the VMRUN is rejected when the guest state has
> CR0.PG=0, EFER.LMA=0 and CR3[63:32] != 0, a combination that bare-metal
> SVM does not reject.  Is that check intentional?  If it is not, a
> Hyper-V fix would help older KVM versions as well; either way the patch
> avoids creating that state in the first place.
> 
> The full patch is here:
> https://lore.kernel.org/r/20260928-kvm-smm-cr3-upper-bits-v1-1-138f52dd531e@gmail.com

Please share the Windows version from `winver`.

My colleague said that it had been fixed. Maybe the change needs to be ported.

Thanks,
Wei

> 
> Thanks,
> Qiliang

Re: [PATCH] KVM: x86: Clear CR3[63:32] on SMM entry from long mode

[Sean Christopherson]

On Mon, Sep 28, 2026, Wei Liu wrote:
> Hi Qiliang
> 
> On Mon, Sep 28, 2026 at 03:25:34PM +0800, Qiliang Yuan wrote:
> > +Cc Hyper-V folks and linux-hyperv, since the failure is triggered by
> > Hyper-V's handling of VMRUN for a nested (L1) hypervisor, i.e. KVM
> > running on WSL2.
> > 
> > For the Hyper-V side: the VMRUN is rejected when the guest state has
> > CR0.PG=0, EFER.LMA=0 and CR3[63:32] != 0, a combination that bare-metal
> > SVM does not reject.  Is that check intentional?  If it is not, a
> > Hyper-V fix would help older KVM versions as well; either way the patch
> > avoids creating that state in the first place.
> > 
> > The full patch is here:
> > https://lore.kernel.org/r/20260928-kvm-smm-cr3-upper-bits-v1-1-138f52dd531e@gmail.com
> 
> Please share the Windows version from `winver`.
> 
> My colleague said that it had been fixed. Maybe the change needs to be ported.

Ya, I'd much prefer to get this fixed in Hyper-V.  If we need to carry a KVM hack,
then at the very least it should be limited to when KVM detects that it's running
on Hyper-V. 

Re: [PATCH] KVM: x86: Clear CR3[63:32] on SMM entry from long mode

[Qiliang Yuan]

On Mon, Sep 28, 2026 at 10:42:43AM -0700, Wei Liu wrote:
> Please share the Windows version from `winver`.
>
> My colleague said that it had been fixed. Maybe the change needs to be ported.

Windows 11 Pro, version 25H2, OS build 26200.9457, with WSL 2.9.3.0.
The host CPU is an AMD Ryzen 9 7940HX.

This machine is fully up to date through Windows Update, and the failure
reproduces on this build, so the fix does not seem to have reached the
released builds yet.  Which build or channel (e.g. Insider) carries it?
I am happy to test it.

I have also filed it as a WSL issue, with repro steps and the SMRAM
analysis: https://github.com/microsoft/WSL/issues/41709

On Mon, Sep 28, 2026 at 11:19:21AM -0700, Sean Christopherson wrote:
> Ya, I'd much prefer to get this fixed in Hyper-V.  If we need to carry a KVM hack,
> then at the very least it should be limited to when KVM detects that it's running
> on Hyper-V.

Understood, a Hyper-V fix is the right answer.  Since current released
builds are affected, I can send a v2 that applies the workaround only
when KVM is running on Hyper-V, if you'd take that in the meantime.
Otherwise I'll drop this and wait for the Hyper-V fix to ship.

Thanks,
Qiliang

Re: [PATCH] KVM: x86: Clear CR3[63:32] on SMM entry from long mode

[Wei Liu]

On Tue, Sep 29, 2026 at 08:57:55AM +0800, Qiliang Yuan wrote:
> On Mon, Sep 28, 2026 at 10:42:43AM -0700, Wei Liu wrote:
> > Please share the Windows version from `winver`.
> >
> > My colleague said that it had been fixed. Maybe the change needs to be ported.
> 
> Windows 11 Pro, version 25H2, OS build 26200.9457, with WSL 2.9.3.0.
> The host CPU is an AMD Ryzen 9 7940HX.
> 
> This machine is fully up to date through Windows Update, and the failure
> reproduces on this build, so the fix does not seem to have reached the
> released builds yet.  Which build or channel (e.g. Insider) carries it?
> I am happy to test it.

Windows 11 Pro, version 26H1 should have the fix.

Wei

> 
> I have also filed it as a WSL issue, with repro steps and the SMRAM
> analysis: https://github.com/microsoft/WSL/issues/41709
> 
> On Mon, Sep 28, 2026 at 11:19:21AM -0700, Sean Christopherson wrote:
> > Ya, I'd much prefer to get this fixed in Hyper-V.  If we need to carry a KVM hack,
> > then at the very least it should be limited to when KVM detects that it's running
> > on Hyper-V.
> 
> Understood, a Hyper-V fix is the right answer.  Since current released
> builds are affected, I can send a v2 that applies the workaround only
> when KVM is running on Hyper-V, if you'd take that in the meantime.
> Otherwise I'll drop this and wait for the Hyper-V fix to ship.
> 
> Thanks,
> Qiliang