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

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Date:	Wed, 13 Feb 2013 15:46:19 +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 15:28 +0000, Marc Zyngier wrote:
> >> 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.

The virtio-mmio spec says so because it seemed like a good idea at the
time ;-) after reading the PCI device spec. But - as I said - I missed
the fact that the readl()-like accessors will always do le32_to_cpu().
Apparently ioread32() does the same (there's a separate ioread32be()).
So I'm not sure that the spec is correct in this aspect any more. Maybe
it should specify the registers as LE always, similarly to PCI? This
problem is already covered by "2.3.1 A Note on Virtqueue Endianness" in
the spec...

> > 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...

Do the other registers like queuenum make sense? Could it be that the
page number of the ring you're getting has wrong endianness?

Paweł


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