Re: [Xen-devel] vmx_disable_intercept_for_msr(v, MSR_SHADOW_GS_BASE)

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/07/13 10:51, Keir Fraser wrote:
> On 01/07/2013 10:44, "Andrew Cooper" <andrew.cooper3@xxxxxxxxxx> wrote:
>
>> On 01/07/13 10:38, Jan Beulich wrote:
>>>>>> On 28.06.13 at 23:02, Cyclonus J <cyclonusj@xxxxxxxxx> wrote:
>>>> I am wondering if we can disable the VMX interception for
>>>> MSR_SHADOW_GS_BASE as AMD is already doing that.
>>> I can't immediately see any reason why we shouldn't be permitted
>>> to do this, but I also don't think this should be performance critical.
>>>
>>> If you feel this is important, why don't you contribute a patch,
>>> with its description saying under what conditions this can yield
>>> measurable benefit?
>>>
>>> Jan
>> Will this not cause a VMexit on each swapgs instruction, as the
>> instruction itself does write to MSR 0xC0000102?
>>
>> I have looked quite closely through the Intel manuals and cant find
>> confirmation one way or another.
> Only RDMSR/WRMSR trap on the MSR bitmaps.

Ok - so the performance aspect depends on whether the guest is using
per-thread kernel areas or not, in combination with swapgs.

I would have thought that this would be a sensible change to make at the
start of 4.4

~Andrew

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