Re: [Qemu-devel] [PATCH v2] arm: add fw_cfg to "virt" board

[Richard W.M. Jones]

On Tue, Dec 09, 2014 at 03:47:44PM +0000, Peter Maydell wrote:
> On 9 December 2014 at 15:41, Laszlo Ersek <address@hidden> wrote:
> > Again, this was the idea that Rich had in 2010 (see the links in the
> > discussion thus far). It was rejected back then (which is why I didn't
> > even try to resurrect it now), and Peter has now asked if anything has
> > changed that would make that approach more acceptable now.
> >
> > "Avi and Gleb not hanging around so they can't block the approach now"
> > might or might not be such a change; I don't know. But, I tend to trust
> > their opinion, even if dates back to 2010 in this case.
> 
> Yeah, this is about my point of view. A direct-write-to-memory
> fw_cfg doesn't seem too unreasonable to me, but it would be nice
> to see a solidly argued case from its proponents to counterbalance
> the previous rejection.

The argument for is that it's essentially instantaneous.

This is a big deal for libguestfs where even our new initrd (1.5 MB)
takes a noticable fraction of a second to load, and we want to get our
total start-up time down to 3 seconds to match x86.

> > Anyway I'm certainly not opposed to performance, so if someone can
> > thoroughly refute everything they said in that thread, go ahead.
> 
> As far as I can tell the main line of argument was "if you're
> using fw_cfg to transfer huge amounts of data you're doing
> something wrong".
> 
> Is ARM much higher overhead than x86 for these accesses?

On a slightly related topic, virtio-mmio traps are slow:

https://bugzilla.redhat.com/show_bug.cgi?id=1123555

This also kills libguestfs performance -- throughput this time, rather
than start-up time.

Rich.

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