Messages in this thread | | | Date | Mon, 30 Jun 2008 15:04:33 -0700 | From | "H. Peter Anvin" <> | Subject | Re: Regression: boot failure on AMD Elan TS-5500 |
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Jeremy Fitzhardinge wrote: >> >> Looking at the beginning of startup_32, it seems ds is used before it >> is set: >> >> startup_32: >> cld >> /* test KEEP_SEGMENTS flag to see if the bootloader is asking >> * us to not reload segments */ >> testb $(1<<6), BP_loadflags(%esi) >> jnz 1f >> >> cli >> movl $(__BOOT_DS),%eax >> movl %eax,%ds >> movl %eax,%es >> movl %eax,%fs >> movl %eax,%gs >> movl %eax,%ss >> 1: >> >> Since the testb instruction is a dereference, ds is implicitly used. If >> I move the testb to after "movl %eax,%ds" it seems to work (not that it >> would make any sense there, but just to prove the point). >> >> 1) Am I barking up the wrong tree? >> >> 2) If I'm right I have no idea what the correct solution is; it seems >> that >> a chicken & egg issue is introduced. >> >> Please advise. I am very new to all of this. > > It's a bit odd that the boot loader neglected to set up ds properly, but > changing the testb line to > > testb $(1<<6), %cs:BP_loadflags(%esi) > > should work. (Or perhaps a %ss: override would be better?) > > I'm assuming that the GDT setup isn't completely mad and that the > segments have the same base at least. >
This should have been set up by the *boot code* (specifically lines 57-61 of arch/x86/boot/pmjump.S) since he's using a conventional boot loader (syslinux) so something is utterly fuggled up. Using %cs: here should be safe, though (and *is* more conservative, after all, why otherwise bother reloading these segments at all?), but it still concerns me a great deal if this is broken in this way. It's definitely better than %ss:.
In particular, I'm wondering if the Elan CPU has any strange ordering requirements with regards to the protected mode transition that we're not obeying.
It would be interesting to put in a heavyweight "brutally synchronizing" instruction like WBINVD at various places in arch/x86/boot/pmjump.S and see if it helps.
-hpa
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