Re: [RFC PATCH] virt_mmio: fix signature checking for BE guests

From: Benjamin Herrenschmidt
Date: Thu Feb 14 2013 - 19:08:25 EST

On Wed, 2013-02-13 at 15:28 +0000, Marc Zyngier wrote:

> Well, the spec clearly says that the registers reflect the endianess of
> the guest, and it makes sense: when performing the MMIO access, KVM
> needs to convert between host and guest endianess.

It's actually a horrible idea :-)

What does "guest endianness" means from a qemu perspective if your
emulated CPU can operate in either mode ?

It's actually been causing endless problems, besides linux doesn't have
"Sane" MMIO accessors that say "current endianness". ioreadN/writeN are
LE, realN/writeN are LE, ioreadNbe/iowriteNbe are BE always, the only
"whatever my endianness is" are the __raw ones which also don't have
barriers etc...

> > Having said that, does the change make everything else work with a BE
> > guest? (I assume we're talking about the guest being BE, right? ;-) If
> > so it means that the host is not following the current spec and it
> > treats all the registers as LE.
> Yes, I only care about a BE guest. And no, not much is actually working
> (kvmtool is not happy about the guest addresses it finds in the
> virtio-ring). Need to dive into it and understand what needs to be fixed...
> >> - Reading the MAGIC register byte by byte. Is that allowed? The spec
> >> only says it is 32bit wide.
> >
> > And the spirit of the spec was: _exactly 32bit wide_. It's just simpler
> > to implement one access width on the host side.
> I guessed as much...


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