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

[Gleb Natapov]

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. 

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. 


--
                        Gleb.