[PATCH v16 24/51] KVM: nVMX: Always forward XSAVES/XRSTORS exits from L2 to L1

Sean Christopherson
2025-09-19T22:33:48Z

Unconditionally forward XSAVES/XRSTORS VM-Exits from L2 to L1, as KVM
doesn't utilize the XSS-bitmap (KVM relies on controlling the XSS value
in hardware to prevent unauthorized access to XSAVES state). KVM always
loads vmcs02 with vmcs12's bitmap, and so any exit _must_ be due to
vmcs12's XSS-bitmap.

Drop the comment about XSS never being non-zero in anticipation of
enabling CET_KERNEL and CET_USER support.

Opportunistically WARN if XSAVES is not enabled for L2, as the CPU is
supposed to generate #UD before checking the XSS-bitmap.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/vmx/nested.c | 15 +++++++++------
1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 2156c9a854f4..846c07380eac 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -6570,14 +6570,17 @@ static bool nested_vmx_l1_wants_exit(struct kvm_vcpu *vcpu,
return nested_cpu_has2(vmcs12, SECONDARY_EXEC_WBINVD_EXITING);
case EXIT_REASON_XSETBV:
return true;
- case EXIT_REASON_XSAVES: case EXIT_REASON_XRSTORS:
+ case EXIT_REASON_XSAVES:
+ case EXIT_REASON_XRSTORS:
/*
- * This should never happen, since it is not possible to
- * set XSS to a non-zero value---neither in L1 nor in L2.
- * If if it were, XSS would have to be checked against
- * the XSS exit bitmap in vmcs12.
+ * Always forward XSAVES/XRSTORS to L1 as KVM doesn't utilize
+ * XSS-bitmap, and always loads vmcs02 with vmcs12's XSS-bitmap
+ * verbatim, i.e. any exit is due to L1's bitmap. WARN if
+ * XSAVES isn't enabled, as the CPU is supposed to inject #UD
+ * in that case, before consulting the XSS-bitmap.
*/
- return nested_cpu_has2(vmcs12, SECONDARY_EXEC_ENABLE_XSAVES);
+ WARN_ON_ONCE(!nested_cpu_has2(vmcs12, SECONDARY_EXEC_ENABLE_XSAVES));
+ return true;
case EXIT_REASON_UMWAIT:
case EXIT_REASON_TPAUSE:
return nested_cpu_has2(vmcs12,
--
2.51.0.470.ga7dc726c21-goog