[Lguest] [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

[Avi Kivity]

Avi Kivity wrote:
>
>> Here is a prototype patch of the compressed IRQ stubs -- this patch
>> compresses them down to 7 stubs per 32-byte cache line (or part of cache
>> line) at the expense of a back-to-back jmp which has the potential of
>> being ugly on some pipelines (we can only get 4 stubs into 32 bytes
>> without that).
>>   
>
> You could actually get 4-byte stubs, using a 16-bit call (66 e8 ww 
> ww).  But it would be slower, since we won't be pairing it with a ret.
>
> I suspect we could get it down to three bytes, by sharing the last 
> byte of the four-byte call sequence with the first byte of the next:
>
>  66 e8 ff 66 e8 fc 66 e8 f9 66 e8 f6 ...
>
> Every three bytes a new stub begins; it's a four-byte call to offset 
> 0x6703 relative to the beginning of the first stub.
>
> Can anyone better 24 bits/stub?

I actually got it down to 16 bits: use a 16-bit code segment, so you can 
drop the address size override:

e8 ff e8 fd e8 fb ...

of course the common code has to jump back to a 32-bit code segment.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.