Re: Removing dbregs

[Maxime Villard <max%m00nbsd.net@localhost>]

Le 26/07/2018 à 10:41, Kamil Rytarowski a écrit :
> On 26.07.2018 09:24, Maxime Villard wrote:
> > For the record, I ended up with a functional patch [1]. This removes the
> > reload of DR6/DR7 on each kernel->user transition, and rather does it
> > during context switches, only when dbregs are being used. Tested ATF on
> > i386 and amd64, it works.
> >
> > Kamil wanted to do extra testing, but basically this patch is close to the
> > final version.
> >
> > [1] http://m00nbsd.net/garbage/dbregs/dbregs.diff
>
> It looks good to me. I've tested it under load on i386 and amd64 - for
> the past 3 days - with no regression observed.

Did you test on Xen? I made zero test on xen...

By the way, thinking about it, the performance regression of dbregs on xen
was likely seeable, because touching drX is done via hypercalls. In the
default userret path, we were doing ldr7, ldr6 and rdr6, so three hypercalls
on each user->kernel transition.

Whatever, my patch fixes that.

>  From a style point of view, I would use the ISSET(9)-like API in the
> code, instead of manual bit operations.

For now I'm happier with a &, like the rest of the x86 code.