Re: [PATCH v9 10/27] KVM: x86: Refine xsave-managed guest register/MSR reset handling

[Chao Gao]

On Tue, Jan 23, 2024 at 06:41:43PM -0800, Yang Weijiang wrote:
>Tweak the code a bit to facilitate resetting more xstate components in
>the future, e.g., adding CET's xstate-managed MSRs.
>
>No functional change intended.
>
>Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
>Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
>---
> arch/x86/kvm/x86.c | 15 ++++++++++++---
> 1 file changed, 12 insertions(+), 3 deletions(-)
>
>diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>index 0e7dc3398293..3671f4868d1b 100644
>--- a/arch/x86/kvm/x86.c
>+++ b/arch/x86/kvm/x86.c
>@@ -12205,6 +12205,11 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
> 		static_branch_dec(&kvm_has_noapic_vcpu);
> }
> 
>+static inline bool is_xstate_reset_needed(void)
>+{
>+	return kvm_cpu_cap_has(X86_FEATURE_MPX);
>+}
>+
> void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> {
> 	struct kvm_cpuid_entry2 *cpuid_0x1;
>@@ -12262,7 +12267,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> 	kvm_async_pf_hash_reset(vcpu);
> 	vcpu->arch.apf.halted = false;
> 
>-	if (vcpu->arch.guest_fpu.fpstate && kvm_mpx_supported()) {
>+	if (vcpu->arch.guest_fpu.fpstate && is_xstate_reset_needed()) {
> 		struct fpstate *fpstate = vcpu->arch.guest_fpu.fpstate;
> 
> 		/*
>@@ -12272,8 +12277,12 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> 		if (init_event)
> 			kvm_put_guest_fpu(vcpu);
> 
>-		fpstate_clear_xstate_component(fpstate, XFEATURE_BNDREGS);
>-		fpstate_clear_xstate_component(fpstate, XFEATURE_BNDCSR);
>+		if (kvm_cpu_cap_has(X86_FEATURE_MPX)) {
>+			fpstate_clear_xstate_component(fpstate,
>+						       XFEATURE_BNDREGS);
>+			fpstate_clear_xstate_component(fpstate,
>+						       XFEATURE_BNDCSR);
>+		}

Checking whether KVM supports MPX is indirect and adds complexity.

how about something like:

#define XSTATE_NEED_RESET_MASK		(XFEATURE_MASK_BNDREGS | \
					 XFEATURE_MASK_BNDCSR)

	u64 reset_mask;
	...

	reset_mask = (kvm_caps.supported_xcr0 | kvm_caps.supported_xss) &
			XSTATE_NEED_RESET_MASK;
	if (vcpu->arch.guest_fpu.fpstate && reset_mask) {
	...
		for_each_set_bit(i, &reset_mask, XFEATURE_MAX)
			fpstate_clear_xstate_component(fpstate, i);
	...
	}

then in patch 24, you can simply add CET_U/S into XSTATE_NEED_RESET_MASK.