Re: [Xen-devel] [PATCH] xen/arm64: Don't zero BSS when booting using EFI

[Julien Grall <julien.grall@xxxxxxx>]

On 03/02/17 14:31, Jan Beulich wrote:
> > > > On 03.02.17 at 15:24, <julien.grall@xxxxxxx> wrote:
> > On 03/02/17 07:41, Jan Beulich wrote:
> > > > > > On 02.02.17 at 20:25, <julien.grall@xxxxxxx> wrote:
> > > > @@ -261,6 +265,8 @@ GLOBAL(init_secondary)
> > > >          sub   x20, x19, x0           /* x20 := phys-offset */
> > > >
> > > >          mov   x22, #1                /* x22 := is_secondary_cpu */
> > > > +        /* Skip zero BSS on secondary CPUs to avoid nasty surprises. */
> > > > +        mov   x26, #1                /* X26 := skip_zero_bss */
> > > >
> > > >  common_start:
> > > >          mov   x24, #0                /* x24 := CPU ID. Initialy zero until we
> > > > @@ -314,8 +320,8 @@ common_start:
> > > >
> > > >  el2:    PRINT("- Xen starting at EL2 -\r\n")
> > > >
> > > > -        /* Zero BSS On the boot CPU to avoid nasty surprises */
> > > > -        cbnz  x22, skip_bss
> > > > +        /* Zero BSS only when requested to avoid nasty surprises. */
> > > > +        cbnz  x26, skip_bss
> > > Comparing the original comment here with both this and the
> > > earlier hunk, I think the intended meaning is lost. Zeroing the
> > > BSS on secondary CPUs is certainly a bug, not a nasty surprise.
> > > What I think the original comment is meaning to say is "the
> > > BSS should have been zeroed already, but let's better not rely
> > > on that".
> > This is not the original meaning.
> Are you sure the comment wasn't just copied from x86 code?
Maybe. Regardless that I think the code holds on ARM in both case.

It is a nasty surprises when zero BSS on secondary CPU or EFI because we 
loose all the information.
> > On non-EFI setup BSS will not be
> > zeroed before hand as the loader of Xen does not know the size of BSS.
> So I would recommend correcting the comments at once to reflect
> what they are now supposed to mean.
IHMO, the comment is valid. See why above.

Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel