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

From: Pawel Moll
Date: Wed Feb 13 2013 - 10:08:32 EST

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.


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