Re: [PATCH v4 10/20] KVM:x86: Make guest supervisor states as non-XSAVE managed

[Yang, Weijiang]

On 7/24/2023 4:26 PM, Chao Gao wrote:
> On Thu, Jul 20, 2023 at 11:03:42PM -0400, Yang Weijiang wrote:
> > +static void kvm_save_cet_supervisor_ssp(struct kvm_vcpu *vcpu)
> > +{
> > +	preempt_disable();
> what's the purpose of disabling preemption?

Thanks!

These preempt_disable/enable() becomes unnecessary due to the PLx_SSP 
handling

in sched_in/out(). Will remove them.

> > +	if (unlikely(guest_can_use(vcpu, X86_FEATURE_SHSTK))) {
> > +		rdmsrl(MSR_IA32_PL0_SSP, vcpu->arch.cet_s_ssp[0]);
> > +		rdmsrl(MSR_IA32_PL1_SSP, vcpu->arch.cet_s_ssp[1]);
> > +		rdmsrl(MSR_IA32_PL2_SSP, vcpu->arch.cet_s_ssp[2]);
> > +		/*
> > +		 * Omit reset to host PL{1,2}_SSP because Linux will never use
> > +		 * these MSRs.
> > +		 */
> > +		wrmsrl(MSR_IA32_PL0_SSP, 0);
> You don't need to reset the MSR because current host doesn't enable SSS
> and leaving guest value in the MSR won't affect host behavior.

Yes,  I just want to make the host PLx_SSPs as clean as possible.

> > +	}
> > +	preempt_enable();
> > +}
> > +
> > +static void kvm_reload_cet_supervisor_ssp(struct kvm_vcpu *vcpu)
> > +{
> > +	preempt_disable();
> > +	if (unlikely(guest_can_use(vcpu, X86_FEATURE_SHSTK))) {
> > +		wrmsrl(MSR_IA32_PL0_SSP, vcpu->arch.cet_s_ssp[0]);
> > +		wrmsrl(MSR_IA32_PL1_SSP, vcpu->arch.cet_s_ssp[1]);
> > +		wrmsrl(MSR_IA32_PL2_SSP, vcpu->arch.cet_s_ssp[2]);
> > +	}
> > +	preempt_enable();
> > +}
> save/load PLx_SSP in kvm_sched_in/out() and in VCPU_RUN ioctl is sub-optimal.
>
> How about:
> 1. expose kvm_save/reload_cet_supervisor_ssp()
> 2. reload guest PLx_SSP in {vmx,svm}_prepare_switch_to_guest()
> 3. save guest PLx_SSP in vmx_prepare_switch_to_host() and
>     svm_prepare_host_switch()?
>
> this way existing svm/vmx->guest_state_loaded can help to reduce a lot of
> unnecessary PLx_SSP MSR accesses.

Nice suggestion! It looks workable. I'll try this,  thanks!

> > +
> > int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
> > {
> > 	struct kvm_queued_exception *ex = &vcpu->arch.exception;
> > @@ -11222,6 +11249,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
> > 	kvm_sigset_activate(vcpu);
> > 	kvm_run->flags = 0;
> > 	kvm_load_guest_fpu(vcpu);
> > +	kvm_reload_cet_supervisor_ssp(vcpu);
> >
> > 	kvm_vcpu_srcu_read_lock(vcpu);
> > 	if (unlikely(vcpu->arch.mp_state == KVM_MP_STATE_UNINITIALIZED)) {
> > @@ -11310,6 +11338,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
> > 	r = vcpu_run(vcpu);
> >
> > out:
> > +	kvm_save_cet_supervisor_ssp(vcpu);
> > 	kvm_put_guest_fpu(vcpu);
> > 	if (kvm_run->kvm_valid_regs)
> > 		store_regs(vcpu);
> > @@ -12398,9 +12427,17 @@ void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu)
> > 		pmu->need_cleanup = true;
> > 		kvm_make_request(KVM_REQ_PMU, vcpu);
> > 	}
> > +
> > +	kvm_reload_cet_supervisor_ssp(vcpu);
> > +
> > 	static_call(kvm_x86_sched_in)(vcpu, cpu);
> > }
> >
> > +void kvm_arch_sched_out(struct kvm_vcpu *vcpu, int cpu)
> > +{
> @cpu its meaning isn't clear and isn't used and ...
Yes, I should have removed it.
> > +	kvm_save_cet_supervisor_ssp(vcpu);
> > +}
> > +
> > void kvm_arch_free_vm(struct kvm *kvm)
> > {
> > 	kfree(to_kvm_hv(kvm)->hv_pa_pg);
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index d90331f16db1..b3032a5f0641 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -1423,6 +1423,7 @@ int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
> > int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu);
> >
> > void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu);
> > +void kvm_arch_sched_out(struct kvm_vcpu *vcpu, int cpu);
> >
> > void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu);
> > void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu);
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 66c1447d3c7f..42f28e8905e1 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -5885,6 +5885,7 @@ static void kvm_sched_out(struct preempt_notifier *pn,
> > {
> > 	struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
> >
> > +	kvm_arch_sched_out(vcpu, 0);
> passing 0 always looks problematic.
Can you elaborate? I have no intent to use @cpu now.
> > 	if (current->on_rq) {
> > 		WRITE_ONCE(vcpu->preempted, true);
> > 		WRITE_ONCE(vcpu->ready, true);
> > --
> > 2.27.0