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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, Apr 22, 2014 at 12:19:48PM +0000, Wu, Feng wrote:
> 
> 
> > -----Original Message-----
> > From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> > Sent: Tuesday, April 22, 2014 5:17 PM
> > To: Wu, Feng
> > Cc: Ian.Campbell@xxxxxxxxxx; Dong, Eddie; Nakajima, Jun;
> > xen-devel@xxxxxxxxxxxxx
> > Subject: RE: [PATCH v1 1/6] x86: Add support for STAC/CLAC instructions
> > 
> > >>> 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...
> > 
> 
> Thanks a lot!
> I think I can work on the alternative mechanism after this SMAP patch is 
> finished.

Any time estimates when the alternative patching mechanism would be done? Asking
because it seems to me that we would want that in Xen 4.5 - so need to figure 
out
your timeline to fold it in the release time-frame.

> 
> > Jan
> 
> Thanks,
> Feng
> 
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