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

[Zhang, Chen]

> -----Original Message-----
> From: Yang, Weijiang <weijiang.yang@xxxxxxxxx>
> Sent: Thursday, December 29, 2022 3:03 PM
> To: Zhang, Chen <chen.zhang@xxxxxxxxx>
> Cc: Gao, Chao <chao.gao@xxxxxxxxx>; Pawan Gupta
> <pawan.kumar.gupta@xxxxxxxxxxxxxxx>; Paolo Bonzini
> <pbonzini@xxxxxxxxxx>; Christopherson,, Sean <seanjc@xxxxxxxxxx>; H.
> Peter Anvin <hpa@xxxxxxxxx>; Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>;
> Borislav Petkov <bp@xxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; Thomas
> Gleixner <tglx@xxxxxxxxxxxxx>; x86@xxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
> kvm@xxxxxxxxxxxxxxx
> Subject: Re: [RFC PATCH 6/9] kvm/x86: Add ARCH_CAP_VIRTUAL_ENUM for
> guest MSR_IA32_ARCH_CAPABILITIES
> 
> 
> On 12/29/2022 10:58 AM, Zhang, Chen wrote:
> 
> [...]
> 
> >> \
> >>> +	 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/s
> > oftware-security-guidance/technical-documentation/branch-history-injec
> > tion.html
> 
> If this series running on old platforms, how VMM can set specific vmcs fields,
> 
> e.g., “virtualize IA32_SPEC_CTRL” VM-execution control, to mitigate guest
> issues?

Enable the virtual MSRs does not means to enable the “virtualize IA32_SPEC_CTRL”.
KVM will check "cpu_has_virt_spec_ctrl()" before set specific VMCS.

Thanks
Chen

> 
> 
> >
> >>
> >>> +
> >>>    	/*
> >>>    	 * 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);
> >>>    	}