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KVM: SVM: Handle #MCs in guest outside of fastpath

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Handle Machine Checks (#MC) that happen in the guest (by forwarding them
to the host) outside of KVM's fastpath so that as much host state as
possible is re-loaded before invoking the kernel's #MC handler.  The only
requirement is that KVM invokes the #MC handler before enabling IRQs (and
even that could _probably_ be relaxed to handling #MCs before enabling
preemption).

Waiting to handle #MCs until "more" host state is loaded hardens KVM
against flaws in the #MC handler, which has historically been quite
brittle. E.g. prior to commit 5567d11c21 ("x86/mce: Send #MC singal from
task work"), the #MC code could trigger a schedule() with IRQs and
preemption disabled.  That led to a KVM hack-a-fix in commit 1811d979c7
("x86/kvm: move kvm_load/put_guest_xcr0 into atomic context").

Note, except for #MCs on VM-Enter, VMX already handles #MCs outside of the
fastpath.

Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Reviewed-by: Jon Kohler <jon@nutanix.com>
Link: https://patch.msgid.link/20251118222328.2265758-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson
2025-11-18 14:23:25 -08:00
parent 38ee66cb18
commit ebd1a33655
1 changed files with 9 additions and 9 deletions
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18
arch/x86/kvm/svm/svm.c
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@@ -4325,14 +4325,6 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct kvm_vcpu *vcpu, u64 run_flags)
vcpu->arch.regs_avail &= ~SVM_REGS_LAZY_LOAD_SET;
/*
* We need to handle MC intercepts here before the vcpu has a chance to
* change the physical cpu
*/
if (unlikely(svm->vmcb->control.exit_code ==
SVM_EXIT_EXCP_BASE + MC_VECTOR))
svm_handle_mce(vcpu);
trace_kvm_exit(vcpu, KVM_ISA_SVM);
svm_complete_interrupts(vcpu);
@@ -4621,8 +4613,16 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu)
{
if (to_svm(vcpu)->vmcb->control.exit_code == SVM_EXIT_INTR)
switch (to_svm(vcpu)->vmcb->control.exit_code) {
case SVM_EXIT_EXCP_BASE + MC_VECTOR:
svm_handle_mce(vcpu);
break;
case SVM_EXIT_INTR:
vcpu->arch.at_instruction_boundary = true;
break;
default:
break;
}
}
static void svm_setup_mce(struct kvm_vcpu *vcpu)