Re: [Xen-devel] x86 instruction emulation in hvm

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 07/05/2009 20:39, "Emre Can Sezer" <ecsezer@xxxxxxxx> wrote:

> I'm running an HVM guest using shadow page tables on a 64bit machine.
> I'm working on a project where I mark certain pages read-only and
> capture the writes into these pages.  I then try to emulate the write
> instructions using x86_emulate as is done in arch/x86/mm/shadow/multi.c.
> 
> The instruction I'm trying to emulate is:
> asm("mov %%gs,%0" : "=m" (p->thread.gsindex));
> 
> Since the source operand is a segment register, and the x86_emulate_ops
> structure that is being used does not have a ops->read_segment function
> defined, the emulation fails.
> 
> Is there an easy way to add or activate this functionality?  Perhaps a
> full emulator, since one would expect to see other cases of memory
> writes that are not handled as well.

Easily implemented -- you pass through to hvm_get_segment_register(). My
guess is you'll quickly fault on another instruction which is not so easily
fixed up, however.

 -- Keir



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