Re: [RFC] AMD VM crashing on deferred memory error injection

[Yazen Ghannam]

On Wed, Feb 11, 2026 at 02:42:07AM +0100, William Roche wrote:
> On 2/9/26 22:18, Yazen Ghannam wrote:
> > On Mon, Feb 09, 2026 at 04:08:19PM -0500, Yazen Ghannam wrote:
> > > On Mon, Feb 09, 2026 at 05:36:32PM +0100, William Roche wrote:
> > 
> > [...]
> > 
> > > > According to me, this small kernel fix relies too much on a Qemu AMD
> > > > specific implementation detail.
> > > > 
> > > > Would you have a more appropriate fix to suggest please ?
> > > > 
> > > > Thanks in advance for your feedback.
> > > > William.
> > > 
> > > Thanks William for the report and details.
> > > 
> > > Clearing "STATUS" registers is a normal part of MCA handling.
> > > 
> > > We seem to allow clearing the regular "MCi_STATUS" register. I assume
> > > this gets trapped/ignored by the hypervisor.
> > > 
> > > I expect we need to do the same behavior for the "MCA_DESTAT" register.
> > > 
> > > I'll do some research here, but please do share any pointers you may
> > > have.
> 
> Yazen, I'm simply trying to find an answer in the AMD64 Architecture
> Programmer's Manual, Volume 2: System Programming, 24593
> 
> This documents indicates (In chapter 9.3.3.4 MCA Deferred Error Status
> Register) that:
> "When the deferred error has been processed by the deferred error handler,
> MCA_DESTAT should be
> cleared. If MCA_STATUS also contains a deferred error, MCA_STATUS should be
> cleared."
> 
> So I would imagine that allowing the reset of MCA_DESTAT the same way as
> MCA_STATUS should be what the platform has to allow (or ignore).
> 

Yes, that's what I gathered too.

> > 
> > Sorry for the rapid reply, but I think this is where we need an update.
> > 
> > Linux:
> > arch/x86/kvm/x86.c : set_msr_mce()
> > 
> > Please note the comment:
> > "All CPUs allow writing 0 to MCi_STATUS MSRs to clear the MSR."
> > 
> > We should include the MCA_DESTAT register range here.
> > 
> > What do you think?
> 
> But before trying to update the set_msr_mce() function, I don't think
> that KVM keeps track of an MSR_AMD64_SMCA_MCx_DESTAT set of registers.
> I can see mce_banks (for ctl, status, addr and misc) and mci_ctl2_banks
> locations in struct kvm_vcpu_arch, but I don't see a location for SMCA
> banks like MCA_DESTAT MSRs.
> 
> So if we make kvm ignore this update instead of raising a #GP error,
> would it be a valid solution ?
> 

Yes, I think so. And the details depend on how much of the platform
needs to be emulated.

Some ideas in increasing order of complexity:

1) Ignore this register write.

2) Do a basic validity check.
   Allow "write 0 to MCA_DESTAT" and #GP for any other value.
   Don't need to save MCA_DESTAT values.

3) Replicate the full platform behavior akin to MCi_STATUS.
   Would need to save MCA_DESTAT values and do a "HWCR" bit check.

I really don't think we want #3. This would useful for "register-based
error injection/simulation"r. But that use case wouldn't do much with the
MCA_DESTAT register without all the related Deferred error interrupt
infrastructure.

So I say the choice is between #1 and #2.

Thanks,
Yazen