Messages in this thread | | | Date | Tue, 01 Jul 2008 13:22:54 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: Regression: boot failure on AMD Elan TS-5500 |
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Sean Young wrote: > On Mon, Jun 30, 2008 at 03:42:47PM -0700, H. Peter Anvin wrote: > >> Jeremy Fitzhardinge wrote: >> >>> Maybe it really does require the far jump immediately after setting PE >>> in cr0... >>> >>> Hm, I don't remember this paragraph being in vol 3a, section 8.9.1 >>> before. Is it a recent addition? >>> >>> Random failures can occur if other instructions exist between steps >>> 3 and 4 above. Failures will be readily seen in some situations, >>> such as when instructions that reference memory are inserted between >>> steps 3 and 4 while in system management mode. >>> >>> >> I don't remember that, either. >> > > Which document are we talking about? >
*The* document ;) http://www.intel.com/products/processor/manuals/ specifically, Volume 3a: System Programming Guide, Part 1.
Section 8.9.1 describes the steps needed to turn on protected mode correctly. It says that you need to do a far jump or call immediately after turning on protected mode. Linux has not done it immediately, and there has been a school of thought that this advice is a workaround for some obsolete CPU, and is not something we have to worry about now.
However, the paragraph I quoted was added since the previous release of the manual, and so presumably documents a current concern. Specifically, the mention of SMM is interesting, because I gather that embedded-class processors like the Elan are very SMM-dependent.
>> Sean: could you try the following patch? >> >> -hpa >> > > >> diff --git a/arch/x86/boot/pmjump.S b/arch/x86/boot/pmjump.S >> index ab049d4..141b6e2 100644 >> --- a/arch/x86/boot/pmjump.S >> +++ b/arch/x86/boot/pmjump.S >> @@ -33,6 +33,8 @@ protected_mode_jump: >> movw %cs, %bx >> shll $4, %ebx >> addl %ebx, 2f >> + jmp 1f # Short jump to serialize on 386/486 >> +1: >> >> movw $__BOOT_DS, %cx >> movw $__BOOT_TSS, %di >> @@ -40,8 +42,6 @@ protected_mode_jump: >> movl %cr0, %edx >> orb $X86_CR0_PE, %dl # Protected mode >> movl %edx, %cr0 >> - jmp 1f # Short jump to serialize on 386/486 >> -1: >> >> # Transition to 32-bit mode >> .byte 0x66, 0xea # ljmpl opcode >> > > I'm afraid it doesn't work. Maybe I can find something in the AMD Elan > documentation. Would a fence make sense? >
Not really, but if it fixes the bug it won't hurt anyone else (unless older processors treat it as an illegal instruction).
J
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