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

[H. Peter Anvin]

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.
> 

Yes, I would consider that a theoretical exercise only :)

> 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?

On the entirely silly level...

CC xx

	-hpa