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SubjectRe: [PATCH v7 00/27] x86, boot, 64bit: Add support for loading ramdisk and bzImage above 4G
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Yes, but why bother...

Borislav Petkov <bp@alien8.de> wrote:

>On Tue, Dec 25, 2012 at 07:51:54AM -0800, H. Peter Anvin wrote:
>> Right, it's the ljmpq issue.
>>
>> Borislav Petkov <bp@alien8.de> wrote:
>>
>> >On Mon, Dec 24, 2012 at 05:06:01PM -0800, H. Peter Anvin wrote:
>> >> Could this be the ljmpq problem that Borislav reported
>> >> (Intel implemented ljmpq, AMD didn't, and I was tempted by a
>> >> micro-optimization which broke AMD which made it into the
>patchset)?
>> >
>> >It has to be. Just booted Yinghai's -v8 in kvm on an AMD host and it
>> >worked fine.
>> >
>> >With the change below it keeps rebooting like I reported earlier.
>I'd
>> >go
>> >out on a limb here and guess that the guest is triple-faulting due
>to
>> >an
>> >unhandled #GP caused by an invalid opcode or similar.
>> >
>> >---
>> >diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
>> >index d94f6d68be2a..1842d30c96a2 100644
>> >--- a/arch/x86/kernel/head_64.S
>> >+++ b/arch/x86/kernel/head_64.S
>> >@@ -279,11 +279,8 @@ ENTRY(secondary_startup_64)
>> > * REX.W + FF /5 JMP m16:64 Jump far, absolute
>indirect,
>> > * address given in m16:64.
>> > */
>> >- movq initial_code(%rip),%rax
>> > pushq $0 # fake return address to stop
>unwinder
>> >- pushq $__KERNEL_CS # set correct cs
>> >- pushq %rax # target address in negative space
>> >- lretq
>> >+ rex64 ljmp *initial_code(%rip)
>> >
>> > #ifdef CONFIG_HOTPLUG_CPU
>
>Btw, I'm being told that we could also do a JMP FAR through a call
>gate in 64-bit mode. Basically, the insn operand contains a CS:OFFSET
>whereas the OFFSET is ignored but only CS is looked at. It has to point
>to a call gate descriptor which contains two pieces of a Code-Segment
>Offset[0:63] and this points into the virtual address space where we
>want to jump to.
>
>Here's a picture to explain it better (figure 4-31 on page 105):
>http://support.amd.com/us/Processor_TechDocs/24593_APM_v2.pdf
>
>It looks like Intel should support that too, AFAICT.

--
Sent from my mobile phone. Please excuse brevity and lack of formatting.


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