[Qemu-ppc] [RFC] QEMU/KVM PowerPC: virtio and guest endianness

[Greg Kurz]

Hi,

There have been some work on the topic lately but no agreement has
been reached yet. I want to consolidate the facts in a single thread of
mail and re-start the discussion. Please find below a recap of what we
have as of today:

From a virtio POV, guest endianness is reflected by the endianness of
the interrupt vectors (ILE bit in the LPCR register). The guest kernel
relies on the H_SET_MODE_RESOURCE_LE hcall to set this bit, early in the
boot process.

Rusty sent a patchset on qemu-devel@ to provide the necessary bits to
perform byteswap in the QEMU:

http://patchwork.ozlabs.org/patch/266451/
http://patchwork.ozlabs.org/patch/266452/
http://patchwork.ozlabs.org/patch/266450/
(plus other enablement patches for virtio drivers, not essential for
the discussion).

In non-KVM mode, QEMU implements the H_SET_MODE_RESOURCE_LE and updates
its internal value for LPCR when the guest requests it. Rusty's patchset
works out-of-the-box in this mode: I could successfully setup and use a
9p share over virtio transport (broader virtio testing still to be done
though).

When using KVM, the story is different : QEMU is not on this
endianness change flow anymore, providing KVM has the following
patch from Anton:

http://patchwork.ozlabs.org/patch/277079/

There are *at least* two approaches to bring back endianness knowledge
to QEMU: polling (1) and propagation (2).

(1) QEMU must retrieve LPCR from the kernel using the following API:

http://patchwork.ozlabs.org/patch/273029/

(2) KVM can resume execution to the host and thus propagating
H_SET_MODE_RESOURCE_LE to QEMU. Laurent came up with a patch on
linuxppc-dev@ to do this:

http://patchwork.ozlabs.org/patch/278590/

I would say (1) is a standard and sane way of addressing the issue:
since the LPCR register value is held by KVM, it makes sense to
introduce an API to get/set it. Then, it is up to QEMU to use this API.

We can dumbly do the polling in all the places where byteswapping
matters: it is clearly sub-optimized, especially since the LPCR_ILE bit
doesn't change so often. Rusty suggested we can retrieve it at virtio
device reset time and cache it, since an endianness change after the
devices have started to be used is non-sensical.

I have searched for an appropriate place to add the polling and I must
admit I did not find any... I am no QEMU expert but I suspect we would
need some kind of arch specific hook to be called from the virtio code
to do this... :-\ I hope I am wrong, please correct me if so.

On the other hand, (2) looks a bit hacky: KVM usually returns to the
host when it cannot fully handle the h_call. Propagating may look like
a useless path to follow from a KVM POV. From a QEMU POV, things are
different: propagation will trig the fallback code in QEMU, already
working in non-KVM mode. Nothing more to be done.

I have a better feeling for (2) because:
- 2-liner patch in KVM
- no extra code change in QEMU
- already *partially* tested

Also, I understood Rusty is working on the next virtio specification
which should address the endian issue: probably not worth to add too
many temporary lines in the QEMU code...

Of course, I probably lack some essential knowledge that would be
more favorable to (1)... so please comment and argue ! :)

Thanks.

--
Greg Kurz