linux-kernel - [PATCH 0/5] x86, fpu/kvm: fix crash with AMX

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Message-ID: <20251224001249.1041934-1-pbonzini@redhat.com>
Date: Wed, 24 Dec 2025 01:12:44 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc: seanjc@...gle.com,
	x86@...nel.org
Subject: [PATCH 0/5] x86, fpu/kvm: fix crash with AMX

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 the host needs enabled to successfully
XRSTOR the guest FPU state.

The first patch replaces inline code in vcpu_enter_guest() with a new
function exported by kernel/fpu.  The new function is similar to
fpregs_lock_and_load() but operates with preemption disabled and
also restores the extra state (currently xfd_err) in struct guest_fpu.

The second patch then introduces a new xfd field in struct guest_fpu,
so that the guest's XFD setting can be swapped while leaving the host
value untouched in fpstate->xfd.

Patches 3 and 4 introduce a test.

Patch 5 makes KVM use fpregs_lock_and_load(), exporting it in place of
two lower-level functions whose other uses are now gone.

Reviews and acks are welcome (this could go in through either
the x86 or KVM trees).

Paolo

Paolo Bonzini (5):
  x86, fpu: introduce fpu_load_guest_fpstate()
  x86, fpu: separate fpstate->xfd and guest XFD
  selftests: kvm: renumber some sync points in amx_test
  selftests, kvm: try getting XFD and XSAVE state out of sync
  KVM: x86: kvm_fpu_get() is fpregs_lock_and_load()

 arch/x86/include/asm/fpu/api.h             |  7 ++--
 arch/x86/include/asm/fpu/types.h           |  7 ++++
 arch/x86/kernel/fpu/core.c                 | 38 ++++++++++-------
 arch/x86/kernel/fpu/xstate.h               | 18 ++++----
 arch/x86/kvm/fpu.h                         |  6 +--
 arch/x86/kvm/x86.c                         | 14 ++-----
 tools/testing/selftests/kvm/x86/amx_test.c | 49 +++++++++++++++-------
 7 files changed, 82 insertions(+), 57 deletions(-)

-- 
2.52.0