On Monday 24 May 2010 21:36:12 Avi Kivity wrote:
> On 05/24/2010 01:03 PM, Sheng Yang wrote:
> > From: Dexuan Cui<[email protected]>
> >
> > Enable XSAVE/XRSTORE for guest.
> >
> > Change from V3:
> > 1. Enforced the assumption that host OS would use all available xstate
> > bits. 2. Various fixes, addressed Avi's comments.
> >
> > I am still not clear about why we need to reload guest xcr0 when
> > cr4.osxsave set...
>
> When cr4.osxsave=0, then the guest executes with the host xcr0 (since
> xgetbv will trap; this is similar to the guest running with the host fpu
> if cr0.ts=0). So if cr4.osxsave transtions, we need to transition xcr0
> as well.
Yes...
>
> > @@ -3354,6 +3356,29 @@ static int handle_wbinvd(struct kvm_vcpu *vcpu)
> >
> > return 1;
> >
> > }
> >
> > +static int handle_xsetbv(struct kvm_vcpu *vcpu)
> > +{
> > + u64 new_bv = kvm_read_edx_eax(vcpu);
> > +
> > + if (kvm_register_read(vcpu, VCPU_REGS_RCX) != 0)
> > + goto err;
> > + if (vmx_get_cpl(vcpu) != 0)
> > + goto err;
> > + if (!(new_bv& XSTATE_FP))
> > + goto err;
> > + if ((new_bv& XSTATE_YMM)&& !(new_bv& XSTATE_SSE))
> > + goto err;
> > + if (new_bv& ~XCNTXT_MASK)
> > + goto err;
>
> Ok. This means we must update kvm immediately when XCNTXT_MASK changes.
>
> (Otherwise we would use KVM_XCNTXT_MASK which is always smaller than
> than XCNTXT_MASK).
I guess use host_xcr0 here is better?
>
> > + vcpu->arch.xcr0 = new_bv;
> > + xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
> > + skip_emulated_instruction(vcpu);
> > + return 1;
> > +err:
> > + kvm_inject_gp(vcpu, 0);
> > + return 1;
> > +}
> > +
> >
> >
> > @@ -4124,6 +4176,8 @@ int kvm_arch_init(void *opaque)
> >
> > perf_register_guest_info_callbacks(&kvm_guest_cbs);
> >
> > + host_xcr0 = xgetbv(XCR_XFEATURE_ENABLED_MASK);
> > +
> >
> > return 0;
>
> Will fault on old cpu.
...
>
> > EXPORT_SYMBOL_GPL(fx_init);
> >
> > @@ -5134,6 +5195,12 @@ void kvm_load_guest_fpu(struct kvm_vcpu *vcpu)
> >
> > vcpu->guest_fpu_loaded = 1;
> > unlazy_fpu(current);
> >
> > + /*
> > + * Restore all possible states in the guest,
> > + * and assume host would use all available bits.
> > + */
> > + if (cpu_has_xsave&& vcpu->arch.xcr0)
> > + xsetbv(XCR_XFEATURE_ENABLED_MASK, host_xcr0);
> >
> > fpu_restore_checking(&vcpu->arch.guest_fpu);
>
> I think we need to reload xcr0 now to the guest's value.
>
> > trace_kvm_fpu(1);
> >
> > }
> >
> > @@ -5144,6 +5211,13 @@ void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
> >
> > return;
> >
> > vcpu->guest_fpu_loaded = 0;
> >
> > + /*
> > + * Save all possible states in the guest,
> > + * and assume host would use all available bits.
> > + * Also load host_xcr0 for host usage.
> > + */
> > + if (cpu_has_xsave&& vcpu->arch.xcr0)
> > + xsetbv(XCR_XFEATURE_ENABLED_MASK, host_xcr0);
> >
> > fpu_save_init(&vcpu->arch.guest_fpu);
> > ++vcpu->stat.fpu_reload;
> > set_bit(KVM_REQ_DEACTIVATE_FPU,&vcpu->requests);
>
> This might be unnecessary.
>
> So far xcr0 life cycle is almost that of
> save_host_state()/load_host_state(), but not exactly. When loading the
> guest fpu we switch temporarily to host xcr0, then we have to switch
> back, but only if gcr4.osxsave. When saving the guest fpu, we're
>
> already using the host xcr0:
> > void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> > {
> >
> > kvm_x86_ops->vcpu_put(vcpu);
> > kvm_put_guest_fpu(vcpu);
> >
> > }
>
> One way to simplify this is to have a vcpu->guest_xcr0_loaded flag and
> check it when needed. So the transition matrix is:
>
> save_host_state: if gcr4.osxsave, set guest_xcr0_loaded, load it
> set gcr4.osxsave: ditto
> clear gcr4.osxsave: do nothing
> load_host_state: if guest_xcr0_loaded, clear it, reload host xcr0
> fpu switching: if (switched) switch; reload fpu; if (switched) switch
>
> may be simplified if we move xcr0 reload back to guest entry (... :)
> but make it lazy:
>
> save_host_state: nothing
> set cr4.osxsave: nothing
> clear cr4.osxsave: nothing
> guest entry: if (gcr4.osxsave && !guest_xcr0_loaded) {
> guest_xcr0_loaded = true, load gxcr0 }
> load_host_state: if (guest_xcr0_loaded) { guest_xcr0_loaded = false;
> load host xcr0 }
> fpu switching: if (guest_xcr0_loaded) { guest_xcr0_loaded = false;
> load host xcr0 }, do fpu stuff
>
> So we delay xcr0 reload as late as possible for both entry and exit.
I think I got it. But why we need do it at "load_host_state()"? I guess just
put
code before fpu testing in kvm_put_guest_fpu() is fine?
--
regards
Yang, Sheng
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