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

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Date:	Wed, 13 Feb 2013 15:08:22 +0000
From:	Pawel Moll <pawel.moll@....com>
To:	Marc Zyngier <Marc.Zyngier@....com>
Cc:	"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 14:25 +0000, Marc Zyngier wrote:
> Using readl() to read the magic value and then memcmp() to check it
> fails on BE, as bytes will be the other way around (by virtue of
> the registers to follow the endianess of the guest).

Hm. Interesting. I missed the fact that readl() as a "PCI operation"
will always assume LE values...

> Fix it by encoding the magic as an integer instead of a string.
> So I'm not completely sure this is the right fix, 

It seems right, however...

> - Using __raw_readl() instead. Is that a generic enough API?
> 
... this implies that either the spec is wrong (as it should say: the
device registers are always LE, in the PCI spirit) or all readl()s & co.
should be replaced with __raw equivalents.

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.

> - 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.

Paweł


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