Re: [PATCH v10 10/27] KVM: x86: Refine xsave-managed guest register/MSR reset handling

[Yang, Weijiang]

On 2/20/2024 11:04 AM, Chao Gao wrote:
> On Sun, Feb 18, 2024 at 11:47:16PM -0800, Yang Weijiang wrote:
> > Tweak the code a bit to facilitate resetting more xstate components in
> > the future, e.g., CET's xstate-managed MSRs.
> >
> > No functional change intended.
> Strictly speaking, there is a functional change. in the previous logic, if
> either of BNDCSR or BNDREGS state is not supported (kvm_mpx_supported() will
> return false), KVM won't reset either of them.
> Since this gets changed, I vote
> to drop 'No functional change ..'

Yes, I'll remove it since existing logic is slightly changed.

> > Suggested-by: Chao Gao <chao.gao@xxxxxxxxx>
> > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > ---
> > arch/x86/kvm/x86.c | 30 +++++++++++++++++++++++++++---
> > 1 file changed, 27 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 10847e1cc413..5a9c07751c0e 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -12217,11 +12217,27 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
> > 		static_branch_dec(&kvm_has_noapic_vcpu);
> > }
> >
> > +#define XSTATE_NEED_RESET_MASK	(XFEATURE_MASK_BNDREGS | \
> > +				 XFEATURE_MASK_BNDCSR)
> > +
> > +static bool kvm_vcpu_has_xstate(unsigned long xfeature)
> kvm_vcpu_has_xstate is a misnomer because it doesn't take a vCPU.

True, I'll change it, thanks!

> > +{
> > +	switch (xfeature) {
> > +	case XFEATURE_MASK_BNDREGS:
> > +	case XFEATURE_MASK_BNDCSR:
> > +		return kvm_cpu_cap_has(X86_FEATURE_MPX);
> > +	default:
> > +		return false;
> > +	}
> > +}
> > +
> > void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> > {
> > 	struct kvm_cpuid_entry2 *cpuid_0x1;
> > 	unsigned long old_cr0 = kvm_read_cr0(vcpu);
> > +	DECLARE_BITMAP(reset_mask, 64);
> > 	unsigned long new_cr0;
> > +	unsigned int i;
> >
> > 	/*
> > 	 * Several of the "set" flows, e.g. ->set_cr0(), read other registers
> > @@ -12274,7 +12290,12 @@ 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()) {
> > +	bitmap_from_u64(reset_mask, (kvm_caps.supported_xcr0 |
> > +				     kvm_caps.supported_xss) &
> > +				    XSTATE_NEED_RESET_MASK);
> > +
> > +	if (vcpu->arch.guest_fpu.fpstate &&
> > +	    !bitmap_empty(reset_mask, XFEATURE_MAX)) {
> > 		struct fpstate *fpstate = vcpu->arch.guest_fpu.fpstate;
> >
> > 		/*
> > @@ -12284,8 +12305,11 @@ 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);
> > +		for_each_set_bit(i, reset_mask, XFEATURE_MAX) {
> > +			if (!kvm_vcpu_has_xstate(i))
> > +				continue;
> The kvm_vcpu_has_xstate() check is superfluous because @i is derived from
> kvm_caps.supported_xcr0/xss, which already guarantees that all unsupported
> xfeatures are filtered out.

Yeah, at least currently I can skip the check for CET/MPX feaures, will remove it, thanks!

> I recommend dropping this check. w/ this change,
>
> Reviewed-by: Chao Gao <chao.gao@xxxxxxxxx>
>
> > +			fpstate_clear_xstate_component(fpstate, i);
> > +		}
> >
> > 		if (init_event)
> > 			kvm_load_guest_fpu(vcpu);
> > --
> > 2.43.0