Re: [PATCH 0/3] virtio-mmio: handle BE guests on LE hosts

Michael S. Tsirkin
2013-10-14T14:03:37Z

On Mon, Oct 14, 2013 at 02:49:10PM +0100, Marc Zyngier wrote:
> On 14/10/13 14:39, Alexander Graf wrote:
> >
> > On 14.10.2013, at 15:24, Marc Zyngier <marc.zyngier@arm.com> wrote:
> >
> >> On 14/10/13 14:10, Alexander Graf wrote:
> >>>
> >>> On 14.10.2013, at 15:03, Paolo Bonzini <pbonzini@redhat.com> wrote:
> >>>
> >>>> Il 11/10/2013 16:36, Marc Zyngier ha scritto:
> >>>>> This small patch series adds just enough kernel infrastructure and
> >>>>> fixes to allow a BE guest to use virtio-mmio on a LE host, provided
> >>>>> that the host actually supports such madness.
> >>>>
> >>>> More precisely, it allows the guest drivers to pick the endianness they
> >>>> prefer. Mixed-endian virtio works fine on QEMU with e.g. a mips guest
> >>>> in emulation mode, because then any given QEMU binary will always use
> >>>> the same endianness (e.g. big for qemu-system-mips).
> >>>
> >>> We have the same problem (runtime switchable endianness) on PowerPC. IBM POWER is gaining Little Endian support in Linux now, so we could easily end up with an LE guest on a BE host.
> >>>
> >>> IIRC the way we're going to solve this is to hack up virtio_is_big_endian() to evaluate the first CPU's endianness mode (which will always be the same as all other CPU's endianness mode due to hypercall restrictions).
> >>
> >> I have implemented something similar for MMIO emulation in KVM/arm
> >> (except that I only care about the faulting CPU).
> >>
> >> See my initial patch for that:
> >> https://lists.cs.columbia.edu/pipermail/kvmarm/2013-October/007359.html
> >>
> >> That doesn't really change the non-trapping virtio accesses, though.
> >> Where is this virtio_is_big_endian() thing?
> >
> > It's in QEMU's exec.c. It only gets used for config space access that goes through PCI though. Is there any other place where virtio specifies native endianness today?
>
> That's the main problem. Today's virtio flavour doesn't specify anything
> about endianness, and that is what I'm adding. Or rather (as Paolo put
> it), the prefered endianness of the virtio driver.
>
> So once (and if) this flags are in place, you always know what you're
> dealing with. And because it is virtio-centric, you can implement it in
> an architecture independent way.
>
> Also, most of my life revolves around kvmtool. QEMU is hardly on my
> radar, these days (for reasons that are neither technical, nor relevant
> to this forum). So it is important to me that the solution is platform
> emulation agnostic.
>
> M.

f you like, you should be able to implement virtio_is_big_endian
in kvmtool too.

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