linux-kernel - Re: [PATCH 30/61] KVM: x86: Handle MPX CPUID adjustment in VMX code

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Date:   Thu, 13 Feb 2020 09:37:17 -0800
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Xiaoyao Li <xiaoyao.li@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 30/61] KVM: x86: Handle MPX CPUID adjustment in VMX code

On Thu, Feb 13, 2020 at 09:51:08PM +0800, Xiaoyao Li wrote:
> On 2/2/2020 2:51 AM, Sean Christopherson wrote:
> >Move the MPX CPUID adjustments into VMX to eliminate an instance of the
> >undesirable "unsigned f_* = *_supported ? F(*) : 0" pattern in the
> >common CPUID handling code.
> >
> >Note, VMX must manually check for kernel support via
> >boot_cpu_has(X86_FEATURE_MPX).
> 
> Why must?

do_cpuid_7_mask() runs the CPUID result through cpuid_mask(), which masks
features based on boot_cpu_data, i.e. clears bits for features that are
supported by hardware but unsupported/disabled by the kernel.

vmx_set_supported_cpuid() needs to to query boot_cpu_has() to preserve the
"supported by kernel" check provided by cpuid_mask().

> 
> >No functional change intended.
> >
> >Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> >---
> >  arch/x86/kvm/cpuid.c   |  3 +--
> >  arch/x86/kvm/vmx/vmx.c | 14 ++++++++++++--
> >  2 files changed, 13 insertions(+), 4 deletions(-)
> >
> >diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> >index cb5870a323cc..09e24d1d731c 100644
> >--- a/arch/x86/kvm/cpuid.c
> >+++ b/arch/x86/kvm/cpuid.c
> >@@ -340,7 +340,6 @@ static int __do_cpuid_func_emulated(struct kvm_cpuid_array *array, u32 func)
> >  static inline void do_cpuid_7_mask(struct kvm_cpuid_entry2 *entry)
> >  {
> >  	unsigned f_invpcid = kvm_x86_ops->invpcid_supported() ? F(INVPCID) : 0;
> >-	unsigned f_mpx = kvm_mpx_supported() ? F(MPX) : 0;
> >  	unsigned f_umip = kvm_x86_ops->umip_emulated() ? F(UMIP) : 0;
> >  	unsigned f_intel_pt = kvm_x86_ops->pt_supported() ? F(INTEL_PT) : 0;
> >  	unsigned f_la57;
> >@@ -349,7 +348,7 @@ static inline void do_cpuid_7_mask(struct kvm_cpuid_entry2 *entry)
> >  	/* cpuid 7.0.ebx */
> >  	const u32 kvm_cpuid_7_0_ebx_x86_features =
> >  		F(FSGSBASE) | F(BMI1) | F(HLE) | F(AVX2) | F(SMEP) |
> >-		F(BMI2) | F(ERMS) | f_invpcid | F(RTM) | f_mpx | F(RDSEED) |
> >+		F(BMI2) | F(ERMS) | f_invpcid | F(RTM) | 0 /*MPX*/ | F(RDSEED) |
> >  		F(ADX) | F(SMAP) | F(AVX512IFMA) | F(AVX512F) | F(AVX512PF) |
> >  		F(AVX512ER) | F(AVX512CD) | F(CLFLUSHOPT) | F(CLWB) | F(AVX512DQ) |
> >  		F(SHA_NI) | F(AVX512BW) | F(AVX512VL) | f_intel_pt;
> >diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> >index 3ff830e2258e..143193fc178e 100644
> >--- a/arch/x86/kvm/vmx/vmx.c
> >+++ b/arch/x86/kvm/vmx/vmx.c
> >@@ -7106,8 +7106,18 @@ static void vmx_cpuid_update(struct kvm_vcpu *vcpu)
> >  static void vmx_set_supported_cpuid(struct kvm_cpuid_entry2 *entry)
> >  {
> >-	if (entry->function == 1 && nested)
> >-		entry->ecx |= feature_bit(VMX);
> >+	switch (entry->function) {
> >+	case 0x1:
> >+		if (nested)
> >+			cpuid_entry_set(entry, X86_FEATURE_VMX);
> >+		break;
> >+	case 0x7:
> >+		if (boot_cpu_has(X86_FEATURE_MPX) && kvm_mpx_supported())
> >+			cpuid_entry_set(entry, X86_FEATURE_MPX);
> >+		break;
> >+	default:
> >+		break;
> >+	}
> >  }
> >  static void vmx_request_immediate_exit(struct kvm_vcpu *vcpu)
> >
>