Messages in this thread | | | Date | Fri, 04 May 2007 07:57:52 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [PATCH] i386: always clear bss |
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Eric W. Biederman wrote: > 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. >>
BTW, I should have marked this as an RFC comment, rather than an actual submission. We don't need it for .22.
> NAK. > > Skipping the segment register load is likely fine. > Supporting V!=P at startup_32 is not. >
Why?
> 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) >
Yes, that's more or less the same code, aside from using 0x40(%esi) as a stack. Would that be OK here?
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