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@xxxxxxx> wrote: > > > >> On 14/10/13 14:10, Alexander Graf wrote: > >>> > >>> On 14.10.2013, at 15:03, Paolo Bonzini <pbonzini@xxxxxxxxxx> 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... -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
