linux-kernel - Re: [PATCH v3 17/27] KVM: x86: Mark CR4.FRED as not reserved when guest can use FRED

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Message-ID: <Zxn0tfA+k4ppu2WL@intel.com>
Date: Thu, 24 Oct 2024 15:18:13 +0800
From: Chao Gao <chao.gao@...el.com>
To: "Xin Li (Intel)" <xin@...or.com>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<linux-doc@...r.kernel.org>, <seanjc@...gle.com>, <pbonzini@...hat.com>,
	<corbet@....net>, <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>, <x86@...nel.org>, <hpa@...or.com>,
	<luto@...nel.org>, <peterz@...radead.org>, <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v3 17/27] KVM: x86: Mark CR4.FRED as not reserved when
 guest can use FRED

>diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>index 03f42b218554..bfdd10773136 100644
>--- a/arch/x86/kvm/vmx/vmx.c
>+++ b/arch/x86/kvm/vmx/vmx.c
>@@ -8009,6 +8009,10 @@ void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> 	kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_LAM);
> 	kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_FRED);
> 
>+	/* Don't allow CR4.FRED=1 before all of FRED KVM support is in place. */
>+	if (!guest_can_use(vcpu, X86_FEATURE_FRED))
>+		vcpu->arch.cr4_guest_rsvd_bits |= X86_CR4_FRED;

is this necessary? __kvm_is_valid_cr4() ensures that guests cannot set any bit
which isn't supported by the hardware.

To account for hardware/KVM caps, I think the following changes will work. This
will fix all other bits besides X86_CR4_FRED.

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 4a93ac1b9be9..2bec3ba8e47d 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1873,6 +1873,7 @@ struct kvm_arch_async_pf {
 extern u32 __read_mostly kvm_nr_uret_msrs;
 extern bool __read_mostly allow_smaller_maxphyaddr;
 extern bool __read_mostly enable_apicv;
+extern u64 __read_mostly cr4_reserved_bits;
 extern struct kvm_x86_ops kvm_x86_ops;
 
 #define kvm_x86_call(func) static_call(kvm_x86_##func)
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 2617be544480..57d82fbcfd3f 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -393,8 +393,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 	vcpu->arch.reserved_gpa_bits = kvm_vcpu_reserved_gpa_bits_raw(vcpu);
 
 	kvm_pmu_refresh(vcpu);
-	vcpu->arch.cr4_guest_rsvd_bits =
-	    __cr4_reserved_bits(guest_cpuid_has, vcpu);
+	vcpu->arch.cr4_guest_rsvd_bits = cr4_reserved_bits |
+					 __cr4_reserved_bits(guest_cpuid_has, vcpu);
 
 	kvm_hv_set_cpuid(vcpu, kvm_cpuid_has_hyperv(vcpu->arch.cpuid_entries,
 						    vcpu->arch.cpuid_nent));
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 34b52b49f5e6..08b42bbd2342 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -119,7 +119,7 @@ u64 __read_mostly efer_reserved_bits = ~((u64)(EFER_SCE | EFER_LME | EFER_LMA));
 static u64 __read_mostly efer_reserved_bits = ~((u64)EFER_SCE);
 #endif
 
-static u64 __read_mostly cr4_reserved_bits = CR4_RESERVED_BITS;
+u64 __read_mostly cr4_reserved_bits;
 
 #define KVM_EXIT_HYPERCALL_VALID_MASK (1 << KVM_HC_MAP_GPA_RANGE)
 
@@ -1110,13 +1110,7 @@ EXPORT_SYMBOL_GPL(kvm_emulate_xsetbv);
 
 bool __kvm_is_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 {
-	if (cr4 & cr4_reserved_bits)
-		return false;
-
-	if (cr4 & vcpu->arch.cr4_guest_rsvd_bits)
-		return false;
-
-	return true;
+	return !(cr4 & vcpu->arch.cr4_guest_rsvd_bits);
 }
 EXPORT_SYMBOL_GPL(__kvm_is_valid_cr4);
 

>+
> 	vmx_setup_uret_msrs(vmx);
> 
> 	if (cpu_has_secondary_exec_ctrls())
>diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
>index 992e73ee2ec5..0ed91512b757 100644
>--- a/arch/x86/kvm/x86.h
>+++ b/arch/x86/kvm/x86.h
>@@ -561,6 +561,8 @@ enum kvm_msr_access {
> 		__reserved_bits |= X86_CR4_PCIDE;       \
> 	if (!__cpu_has(__c, X86_FEATURE_LAM))           \
> 		__reserved_bits |= X86_CR4_LAM_SUP;     \
>+	if (!__cpu_has(__c, X86_FEATURE_FRED))          \
>+		__reserved_bits |= X86_CR4_FRED;        \
> 	__reserved_bits;                                \
> })
> 
>-- 
>2.46.2
>
>