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

Chao Gao
2024-01-25T10:17:40Z

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.