Re: [Qemu-devel] Slow kernel/initrd loading via fw_cfg; Was Re: Hack integrating SeaBios / LinuxBoot option rom with QEMU trace backends

[Gleb Natapov <gleb@redhat.com>]

On Tue, Oct 11, 2011 at 11:59:45AM +0200, Avi Kivity wrote:
> On 10/11/2011 11:56 AM, Gleb Natapov wrote:
> >On Tue, Oct 11, 2011 at 11:49:16AM +0200, Avi Kivity wrote:
> >>  >Whatever we do, the interface will never be as fast as DMA. We will always have to do sanity / permission checks for every IO operation, can batch up only so many IO requests and in QEMU again have to call our callbacks in a loop.
> >>
> >>  We can batch per page, which makes the overhead negligible.
> >>
> >Current code batch userspace exit per 1024 bytes IIRC and changing it to
> >page didn't show significant improvement (also IIRC). But after io data
> >is copied into the kernel emulator process it byte by byte. Possible
> >optimization, which I didn't tried, is to check that destination memory is
> >not mmio and write back the whole buffer if it is the case.
> >
> 
> All the permission checks, segment checks,
> register_address_increment, page table walking, can be done per
> page.  Right now they are done per byte.
> 
Permission checking result is cached in ctxt->perm_ok. I see that
current code check it after several function calls, but this was not the
case before. All others are done for each iteration currently. By writing
back a whole buffer at once we eliminate others too. Interesting how
much it will improve the situation.


> btw Intel also made this optimization, current processors copy
> complete cache lines instead of bytes, so they probably also do the
> checks just once.
> 
> -- 
> error compiling committee.c: too many arguments to function

--
			Gleb.