Re: Single memory slot

[Avi Kivity <avi@redhat.com>]

On 10/16/2009 03:51 AM, Anthony Liguori wrote:
> Avi Kivity wrote:
>> One way to improve the gfn_to_pfn() memslot search is to register 
>> just one slot.  This can only work on 64-bit, since even the smallest 
>> guests need 4GB of physical address space.  Apart from speeding up 
>> gfn_to_page(), it would also speed up mmio which must iterate over 
>> all slots, so a lookup cache cannot help.
>>
>> This would require quite a bunch of changes:
>> - modify gfn_to_pfn() to fail gracefully if the page is in the slot 
>> but unmapped (hole handling)
>> - modify qemu to reserve the guest physical address space
>
> It could potentially speed up qemu quite a lot too as we would return 
> to a model where host va == fixed address + guest pa.  That makes 
> things like stl_phys/ldl_phys trivial.

This doesn't work on 32-bit, and you still need to perform a lookup for 
mmio.  It just shortens the loop.

Note qemu can't depend on mmio holes being unmapped (you could trap the 
SEGV, but that would be unbearably slow).

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