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

[Julien Grall <julien.grall@xxxxxxx>]

Hi Jan,

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. On non-EFI setup BSS will not be 
zeroed before hand as the loader of Xen does not know the size of BSS.
> Another aspect is whether this original meaning of
> the comment actually holds on ARM, since the Xen image is not
> an ELF binary (only xen.axf is).
Cheers,

--
Julien Grall

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