lkml.org 
[lkml]   [2013]   [Feb]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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...



\
 
 \ /
  Last update: 2013-02-13 17:01    [W:0.091 / U:0.048 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site