Re: Single memory slot

[Avi Kivity <avi@redhat.com>]

On 10/16/2009 04:46 AM, Marcelo Tosatti wrote:
> On Thu, Oct 15, 2009 at 04:33:11PM +0900, 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
>> - modify qemu memory allocation to use MAP_FIXED to allocate memory
>> - some hack for the vga aliases (mmap an fd multiple times?)
>> - some hack for the vmx-specific pages (e.g. APIC-access page)
>>
>> Not sure it's worthwhile, but something to keep in mind if a simple
>> cache or sort by size is insufficient due to mmio.
>>      
> Downside is you lose the ability to write protect a small slot only
> (could mprotect(MAP_READ) the desired area but get_log+write_protect
> must be atomic).
>
> Also if you enable dirty log for the large slot largepages are disabled.
>    

I guess that shoots this idea down.  We could perhaps only enable it if 
a vnc client is not connected and we don't track vga updates.

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