Re: [PATCH v2 0/4] x86, fpu/kvm: fix crash with AMX

[Sean Christopherson <seanjc@google.com>]

On Thu, Jan 01, 2026, Paolo Bonzini wrote:
> Fix a possible host panic, due to an unexpected #NM, when a KVM guest
> is using AMX features.
> 
> The guest's XFD value, which is stored in fpstate->xfd, is used for both
> guest execution and host XSAVE operations.  However, the guest-configured
> XFD setting can disable features that were enabled when the guest executed
> XSAVE, and this causes a #NM when executing XRSTOR on the guest FPU state.
> 
> This can happen in two cases: due to a KVM_SET_XSAVE that includes a
> disabled component, or if an interrupt causes XSAVE to be executed
> before the call to fpu_update_guest_xfd().
> 
> The first patch fixes both cases, the rest is improvements to selftests
> in order to cover this test and also verify that #NM faults are injected
> corectly.
> 
> v1 had extra patches to export higher-level functions for KVM in place
> of switch_fpu_return() and fpregs_assert_state_consistent().  Those
> were part of refactoring how KVM loaded guest state when KVM_RUN is
> issued, but are not needed anymore with this v2 fix and I will submit
> them separately.
> 
> Tested on a Sapphire Rapids machine, reviews and acks are welcome so
> that I can submit it to Linus via the KVM tree.

Tested on EMR with with my simulated IRQ hack.  Other than ongoing complaints
about the prints in the selftest, LGTM :-)