Re: x86 instruction emulation in hvm

[Emre Can Sezer <ecsezer@ncsu.edu>]

Yup.  Not only did hvm_get_segment_register() work like a charm, but I 
also ran into another problem as you have foretold.

The instruction is fxsave, which uses a mask to copy some CPU 
information to a 512byte memory.  Any chance of an emulation function 
for this instruction?

As a side note, I know of quite a few research papers that mention 
emulating memory writes to pages, some using Xen.  This leads me to 
believe that the problem of emulating most of these functions should 
have been solved.  I know it's not relevant for Xen production code, but 
I'm wondering if there is a full emulator (perhaps QEMU?) inside Xen 
that I can switch to instead of trying to add these functionalities in 
an ad-hoc manner?

John



Keir Fraser wrote:
> On 07/05/2009 20:39, "Emre Can Sezer" <ecsezer@ncsu.edu> 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
>
>
>