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

[Yang, Weijiang]

On 1/25/2024 6:17 PM, Chao Gao wrote:
> 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.

Yes, the proposal looks good to me, will apply it in next version, thanks!