Re: Should KVM_GUEST stop depending on PARAVIRT?

[Paolo Bonzini <pbonzini@redhat.com>]

On 27/07/2015 17:56, Andy Lutomirski wrote:
> On Mon, Jul 27, 2015 at 6:59 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>>
>> On 24/07/2015 19:33, Andy Lutomirski wrote:
>>> PARAVIRT adds a fair amount of bloat and, AFAICT, KVM_GUEST doesn't
>>> really need any of it.  Would it make sense to drop the dependency?
>>
>> I think the main reason for PARAVIRT is that pv kernels have by default
>>
>>         .read_msr = native_read_msr_safe,
>>         .write_msr = native_write_msr_safe,
>>
>> Unfortunately Intel adds a bunch of performance measurement features
>> saying that "they work with this cpuid family/model/stepping" and at the
>> same time attach them to some non-architectural MSRs that, in principle
>> could be reused for something else years down the road.  This is not a
>> huge problem for Windows, where only tools such as vTune use these MSRs,
>> but it is a problem for Linux.
>>
>> The alternative is ignore_msrs, but that's a very big hammer too.
> 
> I think I'm missing something.  Does KVM_GUEST hook read_msr and
> write_msr?  I don't see it.

PARAVIRT does, and it's the main reason why you'd want PARAVIRT for a
KVM guest.

Paolo

> Xen certainly needs those hooks, but I don't see why KVM would.
> 
> --Andy
>