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

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Date:	Wed, 13 Feb 2013 15:28:52 +0000
From:	Marc Zyngier <marc.zyngier@....com>
To:	Pawel Moll <Pawel.Moll@....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 13/02/13 15:08, Pawel Moll wrote:
> 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.

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.

> 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...

	M.
-- 
Jazz is not dead. It just smells funny...

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