From: "Michael S. Tsirkin" <mst@redhat.com>
To: Marc Zyngier <marc.zyngier@arm.com>
Cc: Pawel Moll <Pawel.Moll@arm.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"virtualization@lists.linux-foundation.org"
<virtualization@lists.linux-foundation.org>,
Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [RFC PATCH] virt_mmio: fix signature checking for BE guests
Date: Wed, 13 Feb 2013 18:53:56 +0200 [thread overview]
Message-ID: <20130213165356.GA23193@redhat.com> (raw)
In-Reply-To: <511BB134.30003@arm.com>
On Wed, Feb 13, 2013 at 03:28:52PM +0000, Marc Zyngier wrote:
> 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...
Does it work for qemu? I know people were using virtio on BE there.
> >> - 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...
next prev parent reply other threads:[~2013-02-13 16:54 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-02-13 14:25 [RFC PATCH] virt_mmio: fix signature checking for BE guests Marc Zyngier
2013-02-13 15:08 ` Pawel Moll
2013-02-13 15:28 ` Marc Zyngier
2013-02-13 15:46 ` Pawel Moll
2013-02-13 16:40 ` Marc Zyngier
2013-02-13 16:53 ` Michael S. Tsirkin [this message]
2013-02-13 17:04 ` Marc Zyngier
2013-02-14 0:22 ` Rusty Russell
2013-02-14 10:54 ` [PATCH] virtio-spec: Define virtio-mmio registers as LE Pawel Moll
2013-02-15 3:57 ` Rusty Russell
2013-03-01 10:41 ` Marc Zyngier
2013-03-01 10:50 ` Pawel Moll
2013-03-01 11:21 ` Marc Zyngier
2013-03-01 12:37 ` Pawel Moll
2013-03-01 13:16 ` Marc Zyngier
2013-03-05 0:11 ` Rusty Russell
2013-03-06 15:10 ` Pawel Moll
2013-03-07 6:36 ` Rusty Russell
2013-02-15 0:07 ` [RFC PATCH] virt_mmio: fix signature checking for BE guests Benjamin Herrenschmidt
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