linux-kernel - Re: [PATCH v10 10/27] KVM: x86: Refine xsave-managed guest register/MSR reset handling

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Message-ID: <119db925-4472-4070-adb3-767f2bd00726@intel.com>
Date: Tue, 20 Feb 2024 21:23:01 +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>,
	<x86@...nel.org>, <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<peterz@...radead.org>, <rick.p.edgecombe@...el.com>, <mlevitsk@...hat.com>,
	<john.allen@....com>
Subject: Re: [PATCH v10 10/27] KVM: x86: Refine xsave-managed guest
 register/MSR reset handling

On 2/20/2024 11:04 AM, Chao Gao wrote:
> On Sun, Feb 18, 2024 at 11:47:16PM -0800, Yang Weijiang wrote:
>> Tweak the code a bit to facilitate resetting more xstate components in
>> the future, e.g., CET's xstate-managed MSRs.
>>
>> No functional change intended.
> Strictly speaking, there is a functional change. in the previous logic, if
> either of BNDCSR or BNDREGS state is not supported (kvm_mpx_supported() will
> return false), KVM won't reset either of them.
> Since this gets changed, I vote
> to drop 'No functional change ..'

Yes, I'll remove it since existing logic is slightly changed.

>> Suggested-by: Chao Gao <chao.gao@...el.com>
>> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
>> ---
>> arch/x86/kvm/x86.c | 30 +++++++++++++++++++++++++++---
>> 1 file changed, 27 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 10847e1cc413..5a9c07751c0e 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -12217,11 +12217,27 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
>> 		static_branch_dec(&kvm_has_noapic_vcpu);
>> }
>>
>> +#define XSTATE_NEED_RESET_MASK	(XFEATURE_MASK_BNDREGS | \
>> +				 XFEATURE_MASK_BNDCSR)
>> +
>> +static bool kvm_vcpu_has_xstate(unsigned long xfeature)
> kvm_vcpu_has_xstate is a misnomer because it doesn't take a vCPU.

True, I'll change it, thanks!

>> +{
>> +	switch (xfeature) {
>> +	case XFEATURE_MASK_BNDREGS:
>> +	case XFEATURE_MASK_BNDCSR:
>> +		return kvm_cpu_cap_has(X86_FEATURE_MPX);
>> +	default:
>> +		return false;
>> +	}
>> +}
>> +
>> void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>> {
>> 	struct kvm_cpuid_entry2 *cpuid_0x1;
>> 	unsigned long old_cr0 = kvm_read_cr0(vcpu);
>> +	DECLARE_BITMAP(reset_mask, 64);
>> 	unsigned long new_cr0;
>> +	unsigned int i;
>>
>> 	/*
>> 	 * Several of the "set" flows, e.g. ->set_cr0(), read other registers
>> @@ -12274,7 +12290,12 @@ 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()) {
>> +	bitmap_from_u64(reset_mask, (kvm_caps.supported_xcr0 |
>> +				     kvm_caps.supported_xss) &
>> +				    XSTATE_NEED_RESET_MASK);
>> +
>> +	if (vcpu->arch.guest_fpu.fpstate &&
>> +	    !bitmap_empty(reset_mask, XFEATURE_MAX)) {
>> 		struct fpstate *fpstate = vcpu->arch.guest_fpu.fpstate;
>>
>> 		/*
>> @@ -12284,8 +12305,11 @@ 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);
>> +		for_each_set_bit(i, reset_mask, XFEATURE_MAX) {
>> +			if (!kvm_vcpu_has_xstate(i))
>> +				continue;
> The kvm_vcpu_has_xstate() check is superfluous because @i is derived from
> kvm_caps.supported_xcr0/xss, which already guarantees that all unsupported
> xfeatures are filtered out.

Yeah, at least currently I can skip the check for CET/MPX feaures, will remove it, thanks!

>
> I recommend dropping this check. w/ this change,
>
> Reviewed-by: Chao Gao <chao.gao@...el.com>
>
>> +			fpstate_clear_xstate_component(fpstate, i);
>> +		}
>>
>> 		if (init_event)
>> 			kvm_load_guest_fpu(vcpu);
>> -- 
>> 2.43.0
>>