Re: [PATCH v4 2/4] x86/boot: Refactor BIOS/PVH start

[Frediano Ziglio <frediano.ziglio@xxxxxxxxx>]

On Thu, Sep 26, 2024 at 7:50 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>
> On 25.09.2024 21:33, Andrew Cooper wrote:
> > On 25/09/2024 7:00 am, Frediano Ziglio wrote:
> >> @@ -449,62 +458,40 @@ __pvh_start:
> >>          mov     %ecx, %es
> >>          mov     %ecx, %ss
> >>
> >> -        /* Skip bootloader setup and bios setup, go straight to 
> >> trampoline */
> >> -        movb    $1, sym_esi(pvh_boot)
> >> -        movb    $1, sym_esi(skip_realmode)
> >> +        /* Load null selector to unused segment registers. */
> >> +        xor     %ecx, %ecx
> >> +        mov     %ecx, %fs
> >> +        mov     %ecx, %gs
> >
> > Honestly, the more I look at this, the more bizarre it is.
> >
> > We should just set up %fs/gs like we do %ds/es, which in this case is
> > simply to drop the comment and the xor.
>
> What's bizarre here? As long as we don't use %fs/%gs, it doesn't matter
> much what we set them to. So yes, they may be set to what %ds etc are set
> to, but they may as well be marked unusable. Documentation-wise that's
> cleaner imo, as down the road - when a need to use one arises - it then
> won't require auditing of all code to figure where the register(s) is(are)
> actually used (just to find: nowhere). Even if a comment to this effect
> was left here, I for one wouldn't trust it in a couple of years time, but
> rather fear it went stale.
>
> Jan

Hi,
  are you against this change and asking to roll it back?

Frediano