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KVM: VMX: Don't modify guest XFD_ERR if CR0.TS=1

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Don't update the guest's XFD_ERR MSR if CR0.TS is set; per the SDM,
XFD_ERR is not modified if CR0.TS=1.  Although it's not explicitly stated
in the SDM, conceptually it makes sense the CR0.TS check would be done
prior to the XFD_ERR check, e.g. CR0.TS=1 blocks all SIMD state, whereas
XFD blocks only XTILE state.

  Device-not-available exceptions that are not due to XFD - those
  resulting from setting CR0.TS to 1 - do not modify the IA32_XFD_ERR MSR.

Opportunistically update the comment to call out that XFD_ERR is updated
before the VM-Exit check occurs.  Nothing in the SDM explicitly calls out
this behavior, but logically it must be the behavior, otherwise reading
XFD_ERR in handle_nm_fault_irqoff() would return stale data, i.e. the
to-be-delivered XFD_ERR value would need to be saved in EXIT_QUALIFICATION,
a la DR6 for #DB and CR2 for #PF, so that software could capture the guest
value.

Fixes: ec5be88ab2 ("kvm: x86: Intercept #NM for saving IA32_XFD_ERR")
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Tested-by: Shan Kang <shan.kang@intel.com>
Link: https://lore.kernel.org/r/20241001050110.3643764-3-xin@zytor.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson 2024-09-30 22:00:45 -07:00
parent a64dcfb451
commit 3ef0df3f76
1 changed files with 8 additions and 8 deletions
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16
arch/x86/kvm/vmx/vmx.c
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@ -6995,16 +6995,16 @@ static void handle_nm_fault_irqoff(struct kvm_vcpu *vcpu)
* MSR value is not clobbered by the host activity before the guest
* has chance to consume it.
*
* Do not blindly read xfd_err here, since this exception might
* be caused by L1 interception on a platform which doesn't
* support xfd at all.
* Update the guest's XFD_ERR if and only if XFD is enabled, as the #NM
* interception may have been caused by L1 interception. Per the SDM,
* XFD_ERR is not modified if CR0.TS=1.
*
* Do it conditionally upon guest_fpu::xfd. xfd_err matters
* only when xfd contains a non-zero value.
*
* Queuing exception is done in vmx_handle_exit. See comment there.
* Note, XFD_ERR is updated _before_ the #NM interception check, i.e.
* unlike CR2 and DR6, the value is not a payload that is attached to
* the #NM exception.
*/
if (vcpu->arch.guest_fpu.fpstate->xfd)
if (vcpu->arch.guest_fpu.fpstate->xfd &&
!kvm_is_cr0_bit_set(vcpu, X86_CR0_TS))
rdmsrl(MSR_IA32_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
}