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

Yang, Weijiang
2022-12-29T07:03:42Z

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/software-security-guidance/technical-documentation/branch-history-injection.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?

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