linux-kernel - Re: [PATCH v9 10/27] KVM: x86: Refine xsave-managed guest register/MSR reset handling

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Date: Fri, 26 Jan 2024 17:13:49 +0800
From: "Yang, Weijiang" <weijiang.yang@...el.com>
To: Chao Gao <chao.gao@...el.com>
CC: <seanjc@...gle.com>, <pbonzini@...hat.com>, <dave.hansen@...el.com>,
	<kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
	<yuan.yao@...ux.intel.com>, <peterz@...radead.org>,
	<rick.p.edgecombe@...el.com>, <mlevitsk@...hat.com>, <john.allen@....com>
Subject: Re: [PATCH v9 10/27] KVM: x86: Refine xsave-managed guest
 register/MSR reset handling

On 1/25/2024 6:17 PM, Chao Gao wrote:
> On Tue, Jan 23, 2024 at 06:41:43PM -0800, Yang Weijiang wrote:
>> Tweak the code a bit to facilitate resetting more xstate components in
>> the future, e.g., adding CET's xstate-managed MSRs.
>>
>> No functional change intended.
>>
>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>> Reviewed-by: Maxim Levitsky <mlevitsk@...hat.com>
>> ---
>> arch/x86/kvm/x86.c | 15 ++++++++++++---
>> 1 file changed, 12 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 0e7dc3398293..3671f4868d1b 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -12205,6 +12205,11 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
>> 		static_branch_dec(&kvm_has_noapic_vcpu);
>> }
>>
>> +static inline bool is_xstate_reset_needed(void)
>> +{
>> +	return kvm_cpu_cap_has(X86_FEATURE_MPX);
>> +}
>> +
>> void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>> {
>> 	struct kvm_cpuid_entry2 *cpuid_0x1;
>> @@ -12262,7 +12267,7 @@ 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()) {
>> +	if (vcpu->arch.guest_fpu.fpstate && is_xstate_reset_needed()) {
>> 		struct fpstate *fpstate = vcpu->arch.guest_fpu.fpstate;
>>
>> 		/*
>> @@ -12272,8 +12277,12 @@ 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);
>> +		if (kvm_cpu_cap_has(X86_FEATURE_MPX)) {
>> +			fpstate_clear_xstate_component(fpstate,
>> +						       XFEATURE_BNDREGS);
>> +			fpstate_clear_xstate_component(fpstate,
>> +						       XFEATURE_BNDCSR);
>> +		}
> Checking whether KVM supports MPX is indirect and adds complexity.
>
> how about something like:
>
> #define XSTATE_NEED_RESET_MASK		(XFEATURE_MASK_BNDREGS | \
> 					 XFEATURE_MASK_BNDCSR)
>
> 	u64 reset_mask;
> 	...
>
> 	reset_mask = (kvm_caps.supported_xcr0 | kvm_caps.supported_xss) &
> 			XSTATE_NEED_RESET_MASK;
> 	if (vcpu->arch.guest_fpu.fpstate && reset_mask) {
> 	...
> 		for_each_set_bit(i, &reset_mask, XFEATURE_MAX)
> 			fpstate_clear_xstate_component(fpstate, i);
> 	...
> 	}
>
> then in patch 24, you can simply add CET_U/S into XSTATE_NEED_RESET_MASK.

Yes, the proposal looks good to me, will apply it in next version, thanks!