Re: [PATCH v3] KVM: Specify byte order for KVM_EXIT_MMIO

[Marc Zyngier <marc.zyngier@arm.com>]

On Fri, Mar 14 2014 at  4:27:56 am GMT, Christoffer Dall <christoffer.dall@linaro.org> wrote:
> On Tue, Jan 28, 2014 at 08:28:42AM -0800, Christoffer Dall wrote:
>> The KVM API documentation is not clear about the semantics of the data
>> field on the mmio struct on the kvm_run struct.
>> 
>> This has become problematic when supporting ARM guests on big-endian
>> host systems with guests of both endianness types, because it is unclear
>> how the data should be exported to user space.
>> 
>> This should not break with existing implementations as all supported
>> existing implementations of known user space applications (QEMU and
>> kvmtools for virtio) only support default endianness of the
>> architectures on the host side.
>> 
>> Cc: Marc Zyngier <marc.zyngier@arm.com>
>> Cc: Peter Maydell <peter.maydell@linaro.org>
>> Cc: Alexander Graf <agraf@suse.de>
>> Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
>> ---
>> Changes [v2 - v3]:
>>  - Change the semantics and wordings as per Scott's, Avi's, and Ben's
>>    suggestions.
>> 
>> Changes [v1 - v2]:
>>  - s/host kernel should/host user space should/
>> 
>>  Documentation/virtual/kvm/api.txt | 4 ++++
>>  1 file changed, 4 insertions(+)
>> 
>> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
>> index 366bf4b..e11f09d 100644
>> --- a/Documentation/virtual/kvm/api.txt
>> +++ b/Documentation/virtual/kvm/api.txt
>> @@ -2565,6 +2565,10 @@ executed a memory-mapped I/O instruction which could not be satisfied
>>  by kvm.  The 'data' member contains the written data if 'is_write' is
>>  true, and should be filled by application code otherwise.
>>  
>> +The 'data' member contains, in its first 'len' bytes, the value as it would
>> +appear if the VCPU performed a load or store of the appropriate width directly
>> +to the byte array.
>> +
>>  NOTE: For KVM_EXIT_IO, KVM_EXIT_MMIO, KVM_EXIT_OSI, KVM_EXIT_DCR,
>>        KVM_EXIT_PAPR and KVM_EXIT_EPR the corresponding
>>  operations are complete (and guest state is consistent) only after userspace
>> -- 
>> 1.8.5.2
>> 
>
> Any more comments on this one, or can it be applied?

I think we should go ahead and apply this. Having dark corners in the
ABI is not a very good thing, and bi-endianness seems to be quite
fashionnable these days... ;-)

Acked-by: Marc Zyngier <marc.zyngier@arm.com>

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