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

Yang, Weijiang
2024-01-26T09:14:12Z

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@intel.com>
>> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.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!