Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2.6.35?

[Richard W.M. Jones]

On Tue, Aug 03, 2010 at 03:37:14PM +0300, Gleb Natapov wrote:
> On Tue, Aug 03, 2010 at 01:10:00PM +0100, Richard W.M. Jones wrote:
> > I can't see anything about this in the kernel changelog.  Can you
> > point me to the commit or the key phrase to look for?
> > 
> 7972995b0c346de76

Thanks - I see.

> > Also, what's the point of making in/out "more correct" when they we
> > know we're talking to qemu (eg. from the CPUID) and we know it already
> > worked fine before with qemu?
> > 
> Qemu has nothing to do with that. ins/outs didn't worked correctly for
> some situation. They didn't work at all if destination/source memory
> was MMIO (didn't work as in hang vcpu IIRC and this is security risk).
> Direction flag wasn't handled at all (if it was set instruction injected
> #GP into a gust). It didn't check that memory it writes to is shadowed
> in which case special action should be taken. It didn't delivered events
> during long string operations. May be more. Unfortunately adding all that
> makes emulation much slower.  I already implemented some speedups, and
> more is possible, but we will not be able to get to previous string io speed
> which was our upper limit.

Thanks for the explanation.  I'll repost my "DMA"-like fw-cfg patch
once I've rebased it and done some more testing.  This huge regression
for a common operation (implementing -initrd) needs to be solved
without using inb/rep ins.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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