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

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Message-ID: <1360886876.22260.9.camel@pasglop>
Date:	Fri, 15 Feb 2013 11:07:56 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	Pawel Moll <Pawel.Moll@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [RFC PATCH] virt_mmio: fix signature checking for BE guests

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...

Cheers,
Ben.


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