Re: [Qemu-devel] [PATCH 05/13] target/arm/kvm: Add kvm_arch_get/put_sve

[Dave Martin]

On Mon, May 13, 2019 at 04:40:52PM +0100, Peter Maydell wrote:
> On Mon, 13 May 2019 at 16:31, Dave Martin <address@hidden> wrote:
> >
> > On Mon, May 13, 2019 at 02:55:01PM +0100, Andrew Jones wrote:
> > > QEMU keeps its 128-bit and larger words in the same order (least
> > > significant word first) for both host endian types. We need to
> > > do word swapping every time we set/get them to/from KVM.
> >
> > I'm not sure whether this is appropriate here, though it depends on
> > what QEMU does with the data.
> 
> The layout is optimised for TCG emulation to be able
> to work with it, I think (rth would have the definite
> reason, though).

So long as we are agreed about the ABI, this is none of my concern :)

> > Something non-obvious to be aware of:
> >
> > As exposed through the signal frame and the KVM ABI, the memory
> > representation of an SVE reg is invariant with respect to the
> > endianness.
> 
> Yes; we handle this conversion as we write out the signal frame:
> https://github.com/qemu/qemu/blob/master/linux-user/aarch64/signal.c#L184

Right.  I hadn't focused consciously on this, since the architecture
does the work for us in the kernel (mostly).

I will check the documentation to make sure the behaviour is clearly
described, anyhow.

Cheers
---Dave