Re: [PATCH v5 05/19] KVM:x86: Initialize kvm_caps.supported_xss

[Yang, Weijiang]

On 8/5/2023 2:45 AM, Sean Christopherson wrote:
> On Thu, Aug 03, 2023, Yang Weijiang wrote:
> > Set kvm_caps.supported_xss to host_xss && KVM XSS mask.
> > host_xss contains the host supported xstate feature bits for thread
> > context switch, KVM_SUPPORTED_XSS includes all KVM enabled XSS feature
> > bits, the operation result represents all KVM supported feature bits.
> > Since the result is subset of host_xss, the related XSAVE-managed MSRs
> > are automatically swapped for guest and host when vCPU exits to
> > userspace.
> >
> > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > ---
> >   arch/x86/kvm/vmx/vmx.c | 1 -
> >   arch/x86/kvm/x86.c     | 6 +++++-
> >   2 files changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 0ecf4be2c6af..c8d9870cfecb 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -7849,7 +7849,6 @@ static __init void vmx_set_cpu_caps(void)
> >   		kvm_cpu_cap_set(X86_FEATURE_UMIP);
> >   
> >   	/* CPUID 0xD.1 */
> > -	kvm_caps.supported_xss = 0;
> Dropping this code in *this* patch is wrong, this belong in whatever patch(es) adds
> IBT and SHSTK support in VMX.
>
> And that does matter because it means this common patch can be carried wih SVM
> support without breaking VMX.
OK, I'll dropping this line for VMX/SVM in CET feature bits enabling patch.
> >   	if (!cpu_has_vmx_xsaves())
> >   		kvm_cpu_cap_clear(X86_FEATURE_XSAVES);
> >   
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 5d6d6fa33e5b..e9f3627d5fdd 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -225,6 +225,8 @@ static struct kvm_user_return_msrs __percpu *user_return_msrs;
> >   				| XFEATURE_MASK_BNDCSR | XFEATURE_MASK_AVX512 \
> >   				| XFEATURE_MASK_PKRU | XFEATURE_MASK_XTILE)
> >   
> > +#define KVM_SUPPORTED_XSS     0
> > +
> >   u64 __read_mostly host_efer;
> >   EXPORT_SYMBOL_GPL(host_efer);
> >   
> > @@ -9498,8 +9500,10 @@ static int __kvm_x86_vendor_init(struct kvm_x86_init_ops *ops)
> >   
> >   	rdmsrl_safe(MSR_EFER, &host_efer);
> >   
> > -	if (boot_cpu_has(X86_FEATURE_XSAVES))
> > +	if (boot_cpu_has(X86_FEATURE_XSAVES)) {
> >   		rdmsrl(MSR_IA32_XSS, host_xss);
> > +		kvm_caps.supported_xss = host_xss & KVM_SUPPORTED_XSS;
> > +	}
> Can you opportunistically (in this patch) hoist this above EFER so that XCR0 and
> XSS are colocated?  I.e. end up with this:
>
> 	if (boot_cpu_has(X86_FEATURE_XSAVE)) {
> 		host_xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
> 		kvm_caps.supported_xcr0 = host_xcr0 & KVM_SUPPORTED_XCR0;
> 	}
> 	if (boot_cpu_has(X86_FEATURE_XSAVES)) {
> 		rdmsrl(MSR_IA32_XSS, host_xss);
> 		kvm_caps.supported_xss = host_xss & KVM_SUPPORTED_XSS;
> 	}
>
> 	rdmsrl_safe(MSR_EFER, &host_efer);
Will change it, thanks!