Re: [PATCH RFC 2/3] x86/kvm/hyper-v: move VMX controls sanitization out of nested_enable_evmcs()

Paolo Bonzini
2020-01-22T14:33:16Z

On 22/01/20 06:47, Sean Christopherson wrote:
>> Yes, it most likely is and it would be nice if Microsoft fixed it, but I
>> guess we're stuck with it for existing Windows versions. Well, for one
>> we found a bug in Hyper-V and not the converse. :)
>>
>> There is a problem with this approach, in that we're stuck with it
>> forever due to live migration. But I guess if in the future eVMCS v2
>> adds an apic_address field we can limit the hack to eVMCS v1. Another
>> possibility is to use the quirks mechanism but it's overkill for now.
>>
>> Unless there are objections, I plan to apply these patches.
> Doesn't applying this patch contradict your earlier opinion? This patch
> would still hide the affected controls from the guest because the host
> controls enlightened_vmcs_enabled.

It does. Unfortunately the key sentence is "we're stuck with it for
existing Windows versions". :(

> Rather than update vmx->nested.msrs or filter vmx_get_msr(), what about
> manually adding eVMCS consistency checks on the disallowed bits and handle
> SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES as a one-off case by simply
> clearing it from the eVMCS? Or alternatively, squashing all the disallowed
> bits.

Hmm, that is also a possibility. It's a very hacky one, but I guess
adding APIC virtualization to eVMCS would require bumping the version to
2. Vitaly, what do you think?

Paolo