Re: [Xen-devel] Real-mode bug with AMD, gPXE, and 32-bit rep movs

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 14:43 +0000 on 26 Mar (1238078627), Keir Fraser wrote:
> Your assertion that it causes GP on Intel is weird. We should be running in
> the emulator already since for the movs to 0x200000 to work we must be
> running in big real mode (i.e., one of the segment registers has a limit
> greater than 0xffff) and so we cannot be emulating that by running the guest
> in vm86 mode.

We do use vm86 mode for big-real-mode; we just clip the segment limits
to 16 bits and carry on, since almost all instructions don't use the big
segments.  Then when we take a fault for the A32 REP MOVS with the
>16-bit offset we go into the emulator and it does the right thing.

Cheers,

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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