On 14/10/13 15:05, Michael S. Tsirkin wrote: > 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. Sure. And I imagine this traps back into the kernel to read some register and find out what the endianness of the accessing CPU is? M. -- 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
