Re: [Qemu-devel] [PATCH v3 3/4] target-ppc: ppc can be either endian

[Alexander Graf <agraf@suse.de>]

On 05/07/2014 12:19 PM, Greg Kurz wrote:
> On Wed, 7 May 2014 11:41:10 +0200
> Alexander Graf <agraf@suse.de> wrote:
>
>>
>>> Am 07.05.2014 um 11:26 schrieb Peter Maydell <peter.maydell@linaro.org>:
>>>
>>>> On 7 May 2014 10:09, Alexander Graf <agraf@suse.de> wrote:
>>>> I don't think we should overengineer hacks for legacy virtio.
>>> Agreed. So what's our final conclusion: virtio endianness
>>> is the endianness of the guest kernel at the point where
>>> it triggers a reset of the virtio device, yes?
>> I just realized we're talking about virtio in a non-virtio thread. This patch set is about core dump support which is different from virtio bi-endian support. While both may end up at the same logic, I don't like the idea to mix them. This function is PPC internal.
>>
>> Alex
>>
> Correct and now I have this feeling about using LPCR_ILE versus MSR_LE...
>
> LPCR_ILE reflects the interrupt vector endianness. It is set during early boot
> by the guest kernel according to the desired endianness. MSR_LE gives the
> current endian mode for the cpu.
>
> The idea is that you need to rely on LPCR_ILE when you peek from the host
> because you lack context and MSR_LE may be have been temporarily changed.
> This is clearly the case for dump support.
>
> Now when it comes to virtio, we cache the endianness at device reset time: MSR_LE from
> current_cpu should reflect the guest kernel endianness, no ?
>
> In this case we could end up like what's being currently discussed with ARM:
>
> http://www.spinics.net/lists/kvm-arm/msg09099.html
> http://www.spinics.net/lists/kvm-arm/msg09091.html
>
> Alex,
>
> If we agree that current_cpu->MSR_LE does the job when the guest kernel resets
> the device, then I guess we don't even need this patch...

Either one works for me as long as we put it into a spec. No solution 
will be able to fulfill all cases.

The uglyness about the current_cpu bit is that devices are usually not 
supposed to know about the cpu accesses come from usually. But then 
again devices shouldn't know about the endianness of a cpu either so I 
guess it's ok to breach layers here.

Rusty, do you have strong feelings either way?


Alex