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

Dave Martin Dave.Martin@arm.com
Wed Jun 12 12:47:00 GMT 2019


On Wed, Jun 12, 2019 at 11:40:11AM +0100, Alex Bennée wrote:
> 
> 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.

Does this issue need looking at for the QEMU userspace emulation too?

Cheers
---Dave



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