linux-kernel - Re: [PATCH v4 09/20] KVM:x86: Add common code of CET MSR access

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Message-ID: <ZMDMQHwlj9m7C39s@chao-email>
Date:   Wed, 26 Jul 2023 15:33:20 +0800
From:   Chao Gao <chao.gao@...el.com>
To:     Yang Weijiang <weijiang.yang@...el.com>
CC:     <seanjc@...gle.com>, <pbonzini@...hat.com>, <peterz@...radead.org>,
        <john.allen@....com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <rick.p.edgecombe@...el.com>,
        <binbin.wu@...ux.intel.com>
Subject: Re: [PATCH v4 09/20] KVM:x86: Add common code of CET MSR access

On Thu, Jul 20, 2023 at 11:03:41PM -0400, Yang Weijiang wrote:
>+static inline bool is_shadow_stack_msr(struct kvm_vcpu *vcpu,

remove @vcpu since it isn't used. And I think it is better to accept
an MSR index than struct msr_data because whether a MSR is a shadow
stack MSR is entirely decided by the MSR index; other fields in the
struct msr_data are irrelevant.

>+				       struct msr_data *msr)
>+{
>+	return msr->index == MSR_IA32_PL0_SSP ||
>+		msr->index == MSR_IA32_PL1_SSP ||
>+		msr->index == MSR_IA32_PL2_SSP ||
>+		msr->index == MSR_IA32_PL3_SSP ||
>+		msr->index == MSR_IA32_INT_SSP_TAB ||
>+		msr->index == MSR_KVM_GUEST_SSP;
>+}
>+
>+static bool kvm_cet_is_msr_accessible(struct kvm_vcpu *vcpu,
>+				      struct msr_data *msr)
>+{
>+
>+	/*
>+	 * This function cannot work without later CET MSR read/write
>+	 * emulation patch.

Probably you should consider merging the "later" patch into this one.
Then you can get rid of this comment and make this patch easier for
review ...

> int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> {
> 	u32 msr = msr_info->index;
>@@ -3982,6 +4023,35 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
> 		vcpu->arch.guest_fpu.xfd_err = data;
> 		break;
> #endif
>+#define CET_IBT_MASK_BITS	GENMASK_ULL(63, 2)

bit9:6 are reserved even if IBT is supported.

>@@ -12131,6 +12217,7 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> 
> 	vcpu->arch.cr3 = 0;
> 	kvm_register_mark_dirty(vcpu, VCPU_EXREG_CR3);
>+	memset(vcpu->arch.cet_s_ssp, 0, sizeof(vcpu->arch.cet_s_ssp));

... this begs the question: where other MSRs are reset. I suppose
U_CET/PL3_SSP are handled when resetting guest FPU. But how about S_CET
and INT_SSP_TAB? there is no answer in this patch.