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

[Paolo Bonzini]

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.

Paolo



Paolo Bonzini (2):
  selftests: kvm: replace numbered sync points with actions
  selftests: kvm: try getting XFD and XSAVE state out of sync

Sean Christopherson (2):
  x86/fpu: Clear XSTATE_BV[i] in save state whenever XFD[i]=1
  selftests: kvm: Verify TILELOADD actually #NM faults when XFD[18]=1

 arch/x86/kernel/fpu/core.c                 |  32 ++++-
 arch/x86/kvm/x86.c                         |   9 ++
 tools/testing/selftests/kvm/x86/amx_test.c | 144 ++++++++++++---------
 3 files changed, 123 insertions(+), 62 deletions(-)

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2.52.0