Re: [Xen-devel] [PATCH] x86: make IDT read-only

["H. Peter Anvin" <hpa@xxxxxxxxx>]

On 04/10/2013 09:31 AM, Eric Northup wrote:
>>
>> If the effect is measurable I agree it is a legitimate optimization.  At
>> one point there was a suggestion to make the code in the IDT vectors
>> differ based on the which interrupt was registed.  While that can also
>> reduce cache misses that can get hairy very quickly, and of course that
>> would require read-write IDTs.
> 
> read-write IDT or GDT are fine: map them twice, once read+write, once
> read-only.  Point the GDTR and IDTR at the read-only alias.
> 

Well, it is weaker, because if you can discover the pointer to the
writable alias you win.

Now, as has been pointed out the GDT needs to be writable on 32 bits as
a matter of hardware requirement.  However, doing it for 64 bits only is
probably enough of a win.

        -hpa



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