Re: [PATCH v3 02/17] KVM: x86/xen: fix Xen hypercall page msr handling

[Vitaly Kuznetsov <vkuznets@redhat.com>]

David Woodhouse <dwmw2@infradead.org> writes:

> On 14 December 2020 21:44:47 GMT, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>>This actually looks more or less like hypercall distinction from after
>>PATCH3:
>>
>>	if (kvm_xen_hypercall_enabled(vcpu->kvm))
>>		return kvm_xen_hypercall(vcpu);
>>
>>        if (kvm_hv_hypercall_enabled(vcpu->kvm))
>>  	        return kvm_hv_hypercall(vcpu);
>>
>>....
>>
>>so my idea was why not do the same for MSRs?
>
> Can you define kvm_hv_msr_enabled()?
>
> Note kvm_hv_hypercall_enabled() is based on a value that gets written
> through the MSR, so it can't be that.

When Hyper-V emulation appeared in KVM we (unfortunately) didn't add a
capability to globaly enable and disable it so to be backwards
compatible we'll have to define kvm_emulating_hyperv() as 'true' for
now as that's how KVM behaves. This, however, doesn't mean we can't add
e.g. a module parameter to disable Hyper-V emulation. Also, we can
probably check guest CPUIDs and if Hyper-V's signature wasn't set we can
return 'false'.

<rant>
Having Hyper-V emulation in KVM 'always enabled' may not be a big deal
from functional point of view but may not be ideal from security
standpoint as bugs in arch/x86/kvm/hyperv.c become exploitable even from
Linux guests.
</rant>

-- 
Vitaly