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

Xiaoyao Li
2025-09-23T14:50:01Z

On 9/20/2025 6:32 AM, Sean Christopherson wrote:
> 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>

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.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,