Re: [PATCH 1/7] KVM: VMX: Allocate MSR Bitmap for each vcpu

[Avi Kivity <avi@redhat.com>]

Sheng Yang wrote:
> On Wednesday 08 October 2008 18:14:22 Avi Kivity wrote:
>   
>> Sheng Yang wrote:
>>     
>>> Separate msr_bitmap for each vcpu, prepared for guest PAT support.
>>>       
>> Why is this necessary?  True, it reduces the overhead of the guest
>> reading and writing the PAT MSRs, but is such access frequent?
>>
>> I would think guests set the PAT once, and never change it later.
>>     
>
> Yeah. In fact, I just think msr_bitmap for each vcpu would be done sooner or 
> later, so get it done here. And it's natural to go with GUEST_PAT. And Xen 
> use it for another purpose(DEBUGCTLMSR related, I haven't checked it through)
> (Also svm.c in KVM use per-vcpu msr bitmap)
>   

svm.c uses it for last branch record; I think these are also rarely 
accessed from the guest.

> And without that, a callback should be implement to hook MSR write and update 
> guest pat write for both vmx and svm, or we should update GUEST PAT every 
> vmentry according to the vcpu->pat. Either way seems not that natural with 
> GUEST_PAT support. 
>   

We need the callbacks (vmx_set_msr and vmx_get_msr, or did you mean 
something else?) anyway for save/restore support.

> However, if you think msr_bitmap for each vcpu is a waste, I'd like to add 
> callbacks.
>   

I agree that we will likely need msr bitmap support one day; but let's 
start without it as this way we test the pat msr callbacks.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.