> Am 07.05.2014 um 11:52 schrieb Marc Zyngier <marc.zyngier@xxxxxxx>:
>
>> On Wed, May 07 2014 at 10:34:30 am BST, Peter Maydell <peter.maydell@xxxxxxxxxx> wrote:
>>> On 6 May 2014 19:38, Peter Maydell <peter.maydell@xxxxxxxxxx> wrote:
>>>> On 6 May 2014 18:25, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
>>>>> On Tue, May 06 2014 at 3:28:07 pm BST, Will Deacon <will.deacon@xxxxxxx> wrote:
>>>>>> On Thu, Apr 24, 2014 at 07:17:23PM +0100, Marc Zyngier wrote:
>>>>>> + reg.addr = (u64)&data;
>>>>>> + if (ioctl(vcpu->vcpu_fd, KVM_GET_ONE_REG, ®) < 0)
>>>>>> + die("KVM_GET_ONE_REG failed (SCTLR_EL1)");
>>>>>> +
>>>>>> + return (data & SCTLR_EL1_EE_MASK) ? VIRTIO_ENDIAN_BE : VIRTIO_ENDIAN_LE;
>>>>>
>>>>> This rules out guests where userspace and kernelspace can run with different
>>>>> endinness. Whilst Linux doesn't currently do this, can we support it here?
>>>>> It all gets a bit hairy if the guest is using a stage-1 SMMU to let
>>>>> userspace play with a virtio device...
>>>>
>>>> Yeah, I suppose we could check either EE or E0 depending on the mode
>>>> when the access was made. We already have all the information, just need
>>>> to handle the case. I'll respin the series.
>>
>>> How virtio implementations should determine their endianness is
>>> a spec question, I think; at any rate QEMU and kvmtool ought to
>>> agree on how it's done. I think the most recent suggestion on the
>>> QEMU mailing list (for PPC) is that we should care about the
>>> guest kernel endianness, but I don't know if anybody thought of
>>> the pass-through-to-userspace usecase...
>>
>> Current opinion on the qemu-devel thread seems to be that we
>> should just define that the endianness of the virtio device is
>> the endianness of the guest kernel at the point where the guest
>> triggers a reset of the virtio device by writing zero the QueuePFN
>> or Status registers.
>
> On AArch32, we only have the CPSR.E bit to select the endiannes. Are we
> going to simply explode if the access comes from userspace?
>
> On AArch64, we can either select the kernel endianness, or userspace
> endianness. Are we going to go a different route just for the sake of
> enforcing kernel access?
>
> I'm inclined to think of userspace access as a valid use case.
It's not for virtio-legacy. It'll be much more productive to influence virtio-1.0 to not redo the same mistakes than enabling even more hackery with the legacy one.
Alex
>
> M.
> --
> Jazz is not dead. It just smells funny.
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