Re: [PATCH 0/5] Fix EFER.NX=0 with EPT

[Avi Kivity <avi@redhat.com>]

On 04/29/2010 02:22 AM, Marcelo Tosatti wrote:
> On Wed, Apr 28, 2010 at 04:47:14PM +0300, Avi Kivity wrote:
>    
>> Currently we run with EFER.NX=1 on the guest even if the guest value is 0.
>> This is fine with shadow, since we check bit 63 when instantiating a page
>> table, and fault if bit 63 is set while EFER.NX is clear.
>>
>> This doesn't work with EPT, since we no longer get the change to check guest
>> ptes.  So we need to run with EFER.NX=0.
>>
>> This is complicated by the fact that if we switch EFER.NX on the host, we'll
>> trap immediately, since some host pages are mapped with the NX bit set.  As
>> a result, we need to switch the MSR atomically during guest entry and exit.
>>
>>      
> Applied, thanks.
>
> Out of curiosity, did you measure the vmentry/vmexit overhead?
>    

Just did now.

   nx=0: 3384
   nx=1: 2203

Perhaps using the dedicated vmcs field is faster (but since it's 
optional, we have to support the generic autoload area).

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.