[kvm-devel] RFC: MMIO endianness flag

[Hollis B. <ho...@us...>]

Add an "is_bigendian" flag to the kvm_run.mmio structure.

This is needed for architectures that can make both little- and
big-endian memory accesses.

Signed-off-by: Hollis Blanchard <ho...@us...>
---

PowerPC has different instructions for native and byte-reversed memory
accesses, and some implementations can also can map individual pages as
byte-reversed. Right now in the PowerPC KVM implementation the kernel
detects byte-reversed MMIO from the guest and converts the data as
appropriate so that userland only ever deals with big-endian data.

That's fine and all, but I started thinking about supporting MMIO
passthrough, in which userland wouldn't emulate an MMIO at all, but
rather execute it on the real hardware (via mmap /dev/mem, for example).

In that case, it's actually very important that the endianness of the
access be preserved, since we need that information to access the real
hardware.

I don't think this patch has any serious x86 ABI implications, since
current x86 code just ignores the flag. I guess x86 could continue to
ignore it in the future, or it could explicitly zero the new flag.

Comments?

diff --git a/include/linux/kvm.h b/include/linux/kvm.h
--- a/include/linux/kvm.h
+++ b/include/linux/kvm.h
@@ -123,6 +123,7 @@ struct kvm_run {
                        __u8  data[8];
                        __u32 len;
                        __u8  is_write;
+                       __u8  is_bigendian;
                } mmio;
                /* KVM_EXIT_HYPERCALL */
                struct {

-- 
Hollis Blanchard
IBM Linux Technology Center