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

[Avi Kivity <avi@redhat.com>]

On 10/11/2011 11:50 AM, Gleb Natapov wrote:
> On Tue, Oct 11, 2011 at 11:26:14AM +0200, Avi Kivity wrote:
> >  rep/ins is exactly like dma+wait for this use case: provide an
> >  address, get a memory image in return.  There's no need to add
> >  another interface, we should just optimize the existing one.
> >
> rep/ins cannot be optimized to be as efficient as dma and remain to
> be correct at the same time. There are various corner cases that
> simplified "fast" implementation will likely miss. Like DF flag
> settings, delaying interrupts for too much, doing ins/outs to/from
> iomem (this is may be not a big problem unless userspace finds a way
> to trigger it). There are ways that current implementation can be
> optimized still though.

These can all go through the slow path, except interrupts, which need to 
be checked after every access.

> But loading MBs of data through fw_cfg interface is just abusing it.
> You wouldn't use pio on real HW to move megabytes of data and expect
> good performance.

True, this is a point in favour of a true dma interface.

-- 
error compiling committee.c: too many arguments to function