[PATCH 0/2] arm64/sve: Fix mutating register endianness on big-endian

Alex Bennée alex.bennee@linaro.org
Wed Jun 12 10:40:00 GMT 2019


Alan Hayward <Alan.Hayward@arm.com> writes:

>> On 7 Jun 2019, at 16:48, Dave Martin <Dave.Martin@arm.com> wrote:
>>
>> On Fri, Jun 07, 2019 at 10:38:58AM +0100, Will Deacon wrote:
>>> On Thu, Jun 06, 2019 at 05:44:53PM +0100, Dave Martin wrote:
>>>> By inspection while debugging something else, I noticed that the byte
>>>> order of FPSIMD V-register stores and SVE Z-register stores is not the
>>>> same when running on big-endian.
>>>>
>>>> This is not properly taken into account when moving between the FPSIMD
>>>> and SVE register views inside the kernel, resulting in the bytes of a
>>>> V-register getting spontaneously reversed in some situations, from
>>>> userspace's point of view.  The signal frame and ptrace interface are
>>>> also affected.  The KVM ABI forbids mixing the two views and so should
>>>> not be affected.
>>>>
>>>> See patch 2 for details.
>>>>
>>>> Patch 1 does some trivial preparatory refactoring.
>>>
>>> Sorry to be a pain, but would you be able to flip this series round so that
>>> the fix doesn't depend on the refactoring, please? That way we can put it
>>> into stable without the dependency.
>>>
>>>> gdb may or may not be affected by this, depending on how it uses the
>>>> NT_PRFPREG and NT_ARM_SVE regsets.  I'll leave it to the developers to
>>>> assess that.
>>>
>>> Wouldn't this be easy enough to test?
>>
>> So, gdb works OK on big-endian but weird stuff happening on both with
>> and without the fix.
>>
>> There are places in the gdb code itself where it is likely missing
>> endianness conversions, but I need to follow up with the gdb folks to
>> clarify whether my patch is missing something…
>
> (I added the SVE support for GDB).
>
> I’ve tried these changes out myself using GDB.
> With your changes everything looks good, apart from:
> * GDB gets it wrong when the ptrace sve structure contains a fpsimd.
> * I need to do some testing around sigcontexts, but again I think GDB
>   will need a slight change.
> I’ll get some patches together for GDB.

Where is the latest state of SVE support for GDB? I really should check
the QEMU gdbstub does the correct things for SVE registers but I was
waiting for upstream gdb support.

--
Alex Bennée



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