Re: [PATCH v3 9/9] kvmtool: virtio: enable arm/arm64 support for bi-endianness

[Marc Zyngier <marc.zyngier@arm.com>]

On 07/05/14 13:17, Peter Maydell wrote:
> On 7 May 2014 12:04, Marc Zyngier <marc.zyngier@arm.com> wrote:
>> On Wed, May 07 2014 at 11:40:54 am BST, Greg Kurz <gkurz@linux.vnet.ibm.com> wrote:
>>> All the fuzz is not really about enforcing kernel access... PPC also
>>> has a current endianness selector (MSR_LE) but it only makes sense
>>> if you are in the cpu context. Initial versions of the virtio biendian
>>> support for QEMU PPC64 used an arbitrary cpu: in this case, the
>>> only sensible thing to look at to support kernel based virtio is the
>>> interrupt endianness selector (LPCR_ILE), because if gives a safe
>>> hint of the kernel endianness.
>>>
>>> The patch set has evolved and now uses current_cpu at device reset time.
>>> As a consequence, we are not necessarily tied to the kernel LPCR_ILE
>>> selector I guess.
> 
> Ah yes, I'd forgotten the history behind why we ended up looking
> at interrupt endianness.
> 
>> That makes a lot of sense, thanks for explaining that. You're basically
>> doing the exact same thing we do with kvmtool on ARM. So if we have
>> similar architectural features on both sides, why don't we support both
>> kernel and userspace access?
> 
> I don't think that we really need to get into whether userspace
> access is or is not a good idea -- "endianness of the CPU which
> does the virtio reset at the point when it does that reset" is a
> nice simple rule that should generalise across architectures,
> so why make it more complicated than that?

This definition looks pretty good to me. Simple and to the point.

Thanks,

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