RE: [Xen-devel] Question about alignment check in sh_x86_emulate_write

["Su, Disheng" <disheng.su@xxxxxxxxx>]

Tim Deegan wrote:
> At 20:38 +0800 on 04 Dec (1196800681), Su, Disheng wrote:
>> In in sh_x86_emulate_write:
>>      if ( vaddr & (bytes-1) )
>>              return X86EMUL_UNHANDLE;
>> I met a problem the vaddr is not aligned on word or dword, but have
>> to emulate it.
> 
> Have you got an OS that does unaligned writes to its pagetables?
> Exciting!  What is it doing?
> 
It does not write to pagetable. I am doing CR0.wp emulation(don't let
guest turn off CR0.wp, but emulate the trapped instructions in shadow).
Some applications like Anti-virus tools will turn off CR0.wp to change
guest IDT etc.
>> I am wordering about the reason for this check. Is it safe to get
>> rid of it?
> 
> Not really.  Among other things, self-aligned writes never cross page
> boundaries.
> 
Is it Ok to add the checking for cross page boundaries instead?
> Tim.



Best Regards,
Disheng, Su

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