On Wed, Aug 23, 2017 at 04:33:18PM +0100, Alex Bennée wrote: > > Dave Martin <Dave.Martin@xxxxxxx> writes: > > > This patch implements the core logic for changing a task's vector > > length on request from userspace. This will be used by the ptrace > > and prctl frontends that are implemented in later patches. > > > > The SVE architecture permits, but does not require, implementations > > to support vector lengths that are not a power of two. To handle > > this, logic is added to check a requested vector length against a > > possibly sparse bitmap of available vector lengths at runtime, so > > that the best supported value can be chosen. > > > > Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx> > > --- > > arch/arm64/include/asm/fpsimd.h | 6 +++ > > arch/arm64/kernel/fpsimd.c | 116 ++++++++++++++++++++++++++++++++++++++++ > > include/uapi/linux/prctl.h | 5 ++ > > 3 files changed, 127 insertions(+) > > > > diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h > > index 7efd04e..39b26d2 100644 > > --- a/arch/arm64/include/asm/fpsimd.h > > +++ b/arch/arm64/include/asm/fpsimd.h > > @@ -70,11 +70,15 @@ extern void fpsimd_update_current_state(struct fpsimd_state *state); > > > > extern void fpsimd_flush_task_state(struct task_struct *target); > > > > +#define SVE_VL_ARCH_MAX 0x100 > > + > > Hmm this isn't the same as SVE_VL_MAX. Why aren't we using that? There should probably be a comment on this. SVE vector-length agnostic software needs to be able to assume that VL <= 256 (the SVE maximum), because some instructions won't work as expected if this is not the case, particularly TBL, where in a larger vector there's no guarantee that a vector index (which might be as small as a byte) can index a vector element (of which there could be >256). I really don't want to expose SVE_VL_ARCH_MAX to userspace, so that people are not tempted to design data structures and protocols that can't handle up to SVE_VL_MAX. Exposing both together seemed to carry a high risk of confusion and misuse. Instead, I silently clamp to an SVE-compatible length (i.e., SVE_VL_ARCH_MAX) in sve_set_vector_length(). If future architecture revisions support larger vectors later on, my plan is to require an extra "force flag" to be passed to override the clamp. [...] Cheers ---Dave
