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

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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@xxxxxxxxx>
Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
---
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@xxxxxxxxx>

+			fpstate_clear_xstate_component(fpstate, i);
+		}

		if (init_event)
			kvm_load_guest_fpu(vcpu);
--
2.43.0






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