VMX: emulated guest use SYSENTER MSRs

VMX: emulated guest use SYSENTER MSRs

[Jan Beulich]

What is the reason for not simply clearing the MSR bitmap bits for these three
MSRs, and rather handling the reads/writes in software?

Jan

Re: VMX: emulated guest use SYSENTER MSRs

[Keir Fraser]

Probably because the existing code must remain anyway (since not all VMX
CPUs support the MSR bitmap), and the benefit of avoiding the intercepts on
CPUs that do support it would have entirely negligible performance benefit,
since the MSRs are written only at start of day. We could add calls to avoid
the intercepts anyway though, if you want to provide a patch.

 -- Keir

On 6/7/07 13:41, "Jan Beulich" <jbeulich@novell.com> wrote:

> What is the reason for not simply clearing the MSR bitmap bits for these three
> MSRs, and rather handling the reads/writes in software?
> 
> Jan
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

RE: VMX: emulated guest use SYSENTER MSRs

[Li, Xin B]

>What is the reason for not simply clearing the MSR bitmap bits 
>for these three MSRs, and rather handling the reads/writes in software?

I haven't checked those MSRs, the original concern is we need
save/restore them when context switch, is this needed?

-Xin 

Re: VMX: emulated guest use SYSENTER MSRs

[Keir Fraser]

Since they are included in the VMCS, and read from there on save/restore,
the vmexit appears to be unnecessary.

 -- Keir

On 7/7/07 08:48, "Li, Xin B" <xin.b.li@intel.com> wrote:

> 
>> What is the reason for not simply clearing the MSR bitmap bits
>> for these three MSRs, and rather handling the reads/writes in software?
> 
> I haven't checked those MSRs, the original concern is we need
> save/restore them when context switch, is this needed?
> 
> -Xin 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

RE: VMX: emulated guest use SYSENTER MSRs

[Li, Xin B]

I will send out the patch soon.

-Xin 

>-----Original Message-----
>From: Keir Fraser [mailto:Keir.Fraser@cl.cam.ac.uk] 
>Sent: Saturday, July 07, 2007 4:52 PM
>To: Li, Xin B; Jan Beulich; xen-devel@lists.xensource.com
>Subject: Re: [Xen-devel] VMX: emulated guest use SYSENTER MSRs
>
>Since they are included in the VMCS, and read from there on 
>save/restore,
>the vmexit appears to be unnecessary.
>
> -- Keir
>
>On 7/7/07 08:48, "Li, Xin B" <xin.b.li@intel.com> wrote:
>
>> 
>>> What is the reason for not simply clearing the MSR bitmap bits
>>> for these three MSRs, and rather handling the reads/writes 
>in software?
>> 
>> I haven't checked those MSRs, the original concern is we need
>> save/restore them when context switch, is this needed?
>> 
>> -Xin 
>> 
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xensource.com
>> http://lists.xensource.com/xen-devel
>

RE: VMX: emulated guest use SYSENTER MSRs

[Jan Beulich]

>>> "Li, Xin B" <xin.b.li@intel.com> 07.07.07 10:57 >>>
>I will send out the patch soon.

Not strictly necessary - I have this as part of a larger patch already. But if you
beat me in getting this out, I'll sync up with yours.

Jan


>-----Original Message-----
>From: Keir Fraser [mailto:Keir.Fraser@cl.cam.ac.uk] 
>Sent: Saturday, July 07, 2007 4:52 PM
>To: Li, Xin B; Jan Beulich; xen-devel@lists.xensource.com 
>Subject: Re: [Xen-devel] VMX: emulated guest use SYSENTER MSRs
>
>Since they are included in the VMCS, and read from there on 
>save/restore,
>the vmexit appears to be unnecessary.
>
> -- Keir
>
>On 7/7/07 08:48, "Li, Xin B" <xin.b.li@intel.com> wrote:
>
>> 
>>> What is the reason for not simply clearing the MSR bitmap bits
>>> for these three MSRs, and rather handling the reads/writes 
>in software?
>> 
>> I haven't checked those MSRs, the original concern is we need
>> save/restore them when context switch, is this needed?
>> 
>> -Xin 
>> 
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xensource.com 
>> http://lists.xensource.com/xen-devel 
>