Re: [Xen-devel] [PATCH v1 1/6] x86: Add support for STAC/CLAC instructions

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.04.14 at 10:46, <feng.wu@xxxxxxxxx> wrote:
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >>> On 22.04.14 at 09:41, <feng.wu@xxxxxxxxx> wrote:
>> > BTW, from the Linux implementation, I think we don't need to check the 
>> > 'cr4'
>> > for the macros, we just need
>> > to check whether the feature exists in the CPU. So is it acceptable to use
>> > the original code by eliminating the cr4 check?
>> 
>> That _might_ be acceptable if you bring it down to just the three
>> really necessary instructions: BT, JNC, CLAC/STAC. But the "might"
>> has to stand - this, after all, remains an addition of a conditional
>> branch (and for the performance of STAC/CLAC I haven't seen any
>> documentation so far either) to several fast paths, and hence the
>> patching alternative can't be discarded as the potentially better one.
>> 
> 
> Since the alternatives mechanism in Linux is something common and 
> independent and needs
> a bit more efforts to be ported to Xen, can we use the method I mentioned 
> above
> at the current stage. After that I will have a fully think about how to port 
> the
> alternatives mechanism Xen. 
> 
> What do you think about this?

Generally this would seem acceptable (as long as you give at least a
rough estimate on when to expect that second step), but then we
have this sad experience with promises by Intel engineers to work
on certain things...

Jan


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