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SubjectRe: [PATCH] i386: always clear bss
Date
Jeremy Fitzhardinge <jeremy@goop.org> writes:

> When the paravirt dispatcher gets run immediately on entry to
> startup_32, the bss isn't cleared. This happens to work if the
> hypervisor's domain builder loaded the complete kernel image and
> cleared the bss for us, but this may not always be true (for example,
> if we're running out of a decompressed bzImage).
>
> Change head.S so that it unconditionally clears the bss before doing
> the paravirt dispatch or continuing on to normal native boot.
>
> There are a couple of points to note:
> - We can't, in general, load the segment registers before paravirt
> dispatch, because we could be running with a non-standard gdt and
> segment selectors. In practice though, all code which ends up
> jumping into startup_32 will have already set the segment registers
> up to sane values, so we don't need to do it again.
> - Paging may or may not be enabled, and if enabled we may or may not
> be mapped to the proper kernel virtual address. To deal with this,
> we compare the kernel's linked address with where we're actually
> running, and use that to offset the bss pointer.

NAK.

Skipping the segment register load is likely fine.
Supporting V!=P at startup_32 is not.
Assuming that we have a stack at startup_32 is not.

If you want to figure out where the kernel is loaded you can do
(from arch/i386/boot/head.S)
>
> /* Calculate the delta between where we were compiled to run
> * at and where we were actually loaded at. This can only be done
> * with a short local call on x86. Nothing else will tell us what
> * address we are running at. The reserved chunk of the real-mode
> * data at 0x34-0x3f are used as the stack for this calculation.
> * Only 4 bytes are needed.
> */
> leal 0x40(%esi), %esp
> call 1f
> 1: popl %ebp
> subl $1b, %ebp
>

Eric
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