RE: [Xen-devel] [Patch] Fix for x86_64 boot failures due to badsegment setup for protected mode.

["Stephen C. Tweedie" <sct@xxxxxxxxxx>]

Hi,

On Fri, 2006-11-10 at 22:39 +0800, Jiang, Yunhong wrote:
> This patch caused one problem.
> 
> If the guest change the DS/SS etc between it set cr0.PE and the jump 
> instruction.

Hmm...  Do you know how much of a problem that causes in practice?
Which guests have that behaviour?

> Considering following code:
>    mov eax, cr0
>    or eax, 0x11
>   mov cr0, eax                ---> here enable PE, and old_ctx.ds is cleard
>    mov ax, 0x0020
>    mov ds, ax         -> changes the DS here

Well, we've got commit log comments for the "pop %es" case:

2) In
current vmxassist, when entering VM86_PROTECTED_TO_REAL, we need to
decode every instruction manually until we catch instructions like
jmpl to reset cs to really return to real mode.

So I'm afraid I just assumed that we wouldn't need to deal with ds in
real life, since we haven't added emulation for that so far.  Is that
emulation we need to add as a matter of some urgency, then?

--Stephen



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