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SubjectRE: [RFC PATCH 6/9] kvm/x86: Add ARCH_CAP_VIRTUAL_ENUM for guest MSR_IA32_ARCH_CAPABILITIES
Date


> -----Original Message-----
> From: Yang, Weijiang <weijiang.yang@intel.com>
> Sent: Wednesday, December 21, 2022 12:04 PM
> To: Zhang, Chen <chen.zhang@intel.com>
> Cc: Gao, Chao <chao.gao@intel.com>; Pawan Gupta
> <pawan.kumar.gupta@linux.intel.com>; Paolo Bonzini
> <pbonzini@redhat.com>; Christopherson,, Sean <seanjc@google.com>; H.
> Peter Anvin <hpa@zytor.com>; Dave Hansen <dave.hansen@linux.intel.com>;
> Borislav Petkov <bp@alien8.de>; Ingo Molnar <mingo@redhat.com>; Thomas
> Gleixner <tglx@linutronix.de>; x86@kernel.org; linux-kernel@vger.kernel.org;
> kvm@vger.kernel.org
> Subject: Re: [RFC PATCH 6/9] kvm/x86: Add ARCH_CAP_VIRTUAL_ENUM for
> guest MSR_IA32_ARCH_CAPABILITIES
>
>
> On 12/11/2022 12:00 AM, Zhang Chen wrote:
> > Add the 63 bit in MSR_IA32_ARCH_CAPABILITIES for enable the virtual MSRs.
> > Virtual MSRs can allow guests to notify VMM whether or not they are
> > using specific software mitigation, allowing a VMM to enable there
> > hardware control only where necessary.
> > As Intel spec defination, expose virtual MSR for guest.
> > Make guest have ability to check virtual MSR 0x50000000.
> >
> > Signed-off-by: Zhang Chen <chen.zhang@intel.com>
> > ---
> > arch/x86/kvm/vmx/vmx.c | 15 +++++++++++++++
> > arch/x86/kvm/vmx/vmx.h | 1 +
> > arch/x86/kvm/x86.c | 16 +++++++++++++++-
> > 3 files changed, 31 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c index
> > 407061b369b4..6ed6b743be0e 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -2001,6 +2001,12 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu,
> struct msr_data *msr_info)
> > case MSR_IA32_DEBUGCTLMSR:
> > msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
> > break;
> > + case MSR_VIRTUAL_ENUMERATION:
> > + if (!msr_info->host_initiated &&
> > + !(vcpu->arch.arch_capabilities &
> ARCH_CAP_VIRTUAL_ENUM))
> > + return 1;
> > + msr_info->data = vmx->msr_virtual_enumeration;
> > + break;
> > default:
> > find_uret_msr:
> > msr = vmx_find_uret_msr(vmx, msr_info->index); @@ -2375,6
> +2381,15
> > @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data
> *msr_info)
> > }
> > ret = kvm_set_msr_common(vcpu, msr_info);
> > break;
> > + case MSR_VIRTUAL_ENUMERATION:
> > + if (msr_info->host_initiated &&
> > + !(vcpu->arch.arch_capabilities &
> ARCH_CAP_VIRTUAL_ENUM))
> > + return 1;
> > + if (data & ~VIRT_ENUM_MITIGATION_CTRL_SUPPORT)
> > + return 1;
> > + vmx->msr_virtual_enumeration = data &
> > +
> VIRT_ENUM_MITIGATION_CTRL_SUPPORT;
> > + break;
> >
> > default:
> > find_uret_msr:
> > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h index
> > c5a41ae14237..fc873cf45f70 100644
> > --- a/arch/x86/kvm/vmx/vmx.h
> > +++ b/arch/x86/kvm/vmx/vmx.h
> > @@ -298,6 +298,7 @@ struct vcpu_vmx {
> > * IA32_SPEC_CTRL_MSR.
> > */
> > u64 spec_ctrl_mask;
> > + u64 msr_virtual_enumeration;
> > u32 msr_ia32_umwait_control;
> >
> > /*
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index
> > 2835bd796639..6be0a3f1281f 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1532,6 +1532,8 @@ static const u32 emulated_msrs_all[] = {
> > MSR_IA32_VMX_EPT_VPID_CAP,
> > MSR_IA32_VMX_VMFUNC,
> >
> > + MSR_VIRTUAL_ENUMERATION,
> > +
> > MSR_K7_HWCR,
> > MSR_KVM_POLL_CONTROL,
> > };
> > @@ -1567,6 +1569,7 @@ static const u32 msr_based_features_all[] = {
> > MSR_IA32_UCODE_REV,
> > MSR_IA32_ARCH_CAPABILITIES,
> > MSR_IA32_PERF_CAPABILITIES,
> > + MSR_VIRTUAL_ENUMERATION,
> > };
> >
> > static u32 msr_based_features[ARRAY_SIZE(msr_based_features_all)];
> > @@ -1588,7 +1591,8 @@ static unsigned int num_msr_based_features;
> > ARCH_CAP_SKIP_VMENTRY_L1DFLUSH | ARCH_CAP_SSB_NO |
> ARCH_CAP_MDS_NO | \
> > ARCH_CAP_PSCHANGE_MC_NO | ARCH_CAP_TSX_CTRL_MSR |
> ARCH_CAP_TAA_NO | \
> > ARCH_CAP_SBDR_SSDP_NO | ARCH_CAP_FBSDP_NO |
> ARCH_CAP_PSDP_NO | \
> > - ARCH_CAP_FB_CLEAR | ARCH_CAP_RRSBA | ARCH_CAP_PBRSB_NO)
> > + ARCH_CAP_FB_CLEAR | ARCH_CAP_RRSBA | ARCH_CAP_PBRSB_NO |
> \
> > + ARCH_CAP_VIRTUAL_ENUM)
> >
> > static u64 kvm_get_arch_capabilities(void)
> > {
> > @@ -1607,6 +1611,13 @@ static u64 kvm_get_arch_capabilities(void)
> > */
> > data |= ARCH_CAP_PSCHANGE_MC_NO;
> >
> > + /*
> > + * Virtual MSRs can allow guests to notify VMM whether or not
> > + * they are using specific software mitigation, allowing a VMM
> > + * to enable there hardware control only where necessary.
> > + */
> > + data |= ARCH_CAP_VIRTUAL_ENUM;
>
>
> IMO, this is:  data &= ARCH_CAP_VIRTUAL_ENUM; because it requires
> platform support.

Intel defined the virtual MSRs for software mitigations for all platforms.
KVM should be unconditionally opened it for the software mitigation in migration pools.
For example migration from the old platform to the new platform.
Please check the Software Mitigations in Migration Pools section in documents:
https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/branch-history-injection.html

>
>
> > +
> > /*
> > * If we're doing cache flushes (either "always" or "cond")
> > * we will do one whenever the guest does a vmlaunch/vmresume.
> > @@ -1657,6 +1668,9 @@ static int kvm_get_msr_feature(struct
> kvm_msr_entry *msr)
> > case MSR_IA32_UCODE_REV:
> > rdmsrl_safe(msr->index, &msr->data);
> > break;
> > + case MSR_VIRTUAL_ENUMERATION:
> > + msr->data = VIRT_ENUM_MITIGATION_CTRL_SUPPORT;
>
>
> Need to check bit 63 of host MSR_ARCH_CAPABILITIES before expose the
> feature.

Refer to the above comments.

Thanks
Chen

>
>
> > + break;
> > default:
> > return static_call(kvm_x86_get_msr_feature)(msr);
> > }
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