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

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Date:   Mon, 24 Jul 2023 21:50:20 +0800
From:   "Yang, Weijiang" <weijiang.yang@...el.com>
To:     Chao Gao <chao.gao@...el.com>
CC:     <seanjc@...gle.com>, <pbonzini@...hat.com>, <peterz@...radead.org>,
        <john.allen@....com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <rick.p.edgecombe@...el.com>,
        <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH v4 10/20] KVM:x86: Make guest supervisor states as
 non-XSAVE managed


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
>>