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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Mon, May 10, 2021 at 01:23:48PM +0100, Mark Brown wrote: > When the SVE vector length is 128 bits then there are no bits in the Z > registers which are not shared with the V registers so we can skip them > when zeroing state not shared with FPSIMD, this results in a minor > performance improvement. > > Signed-off-by: Mark Brown > --- > arch/arm64/include/asm/fpsimd.h | 2 +- > arch/arm64/kernel/entry-fpsimd.S | 9 +++++++-- > arch/arm64/kernel/fpsimd.c | 6 ++++-- > 3 files changed, 12 insertions(+), 5 deletions(-) > > diff --git a/arch/arm64/include/asm/fpsimd.h b/arch/arm64/include/asm/fpsimd.h > index 2599504674b5..c072161d5c65 100644 > --- a/arch/arm64/include/asm/fpsimd.h > +++ b/arch/arm64/include/asm/fpsimd.h > @@ -69,7 +69,7 @@ static inline void *sve_pffr(struct thread_struct *thread) > extern void sve_save_state(void *state, u32 *pfpsr); > extern void sve_load_state(void const *state, u32 const *pfpsr, > unsigned long vq_minus_1); > -extern void sve_flush_live(void); > +extern void sve_flush_live(unsigned long vq_minus_1); > extern void sve_load_from_fpsimd_state(struct user_fpsimd_state const *state, > unsigned long vq_minus_1); > extern unsigned int sve_get_vl(void); > diff --git a/arch/arm64/kernel/entry-fpsimd.S b/arch/arm64/kernel/entry-fpsimd.S > index ee8773f4088b..090449e825e7 100644 > --- a/arch/arm64/kernel/entry-fpsimd.S > +++ b/arch/arm64/kernel/entry-fpsimd.S > @@ -70,10 +70,15 @@ SYM_FUNC_START(sve_load_from_fpsimd_state) > ret > SYM_FUNC_END(sve_load_from_fpsimd_state) > > -/* Zero all SVE registers but the first 128-bits of each vector */ > +/* > + * Zero all SVE registers but the first 128-bits of each vector > + * > + * x0 = VQ - 1 > + */ > SYM_FUNC_START(sve_flush_live) > + cbz x0, 1f // A VQ-1 of 0 is 128 bits so no extra Z state Should we worry about branch mispredicts here? It may be in the noise, but I wonder whether it's worth considering use of alternatives here instead. I have a suspicion that VL = 128 bits won't be common at runtime, except in the case of systems where the physical (or max usable) vector length (i.e., sve_max_vl) is 128 bits. Concerns like this could be addressed later instead though, if/when we have evidence to support changes. > sve_flush_z > - sve_flush_p > +1: sve_flush_p > sve_flush_ffr > ret > SYM_FUNC_END(sve_flush_live) > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c > index ad3dd34a83cf..e57b23f95284 100644 > --- a/arch/arm64/kernel/fpsimd.c > +++ b/arch/arm64/kernel/fpsimd.c > @@ -957,8 +957,10 @@ void do_sve_acc(unsigned int esr, struct pt_regs *regs) > * disabling the trap, otherwise update our in-memory copy. > */ > if (!test_thread_flag(TIF_FOREIGN_FPSTATE)) { > - sve_set_vq(sve_vq_from_vl(current->thread.sve_vl) - 1); > - sve_flush_live(); > + unsigned long vq_minus_one = > + sve_vq_from_vl(current->thread.sve_vl) - 1; > + sve_set_vq(vq_minus_one); > + sve_flush_live(vq_minus_one); Seems reasonable. sve_flush_live() could alternatively be made a C function, with asm wrappers for sve_flush_{z,p,ffr} so that the conditional logic can be inlined -- but I can't see that it would improve the generated code much. So I'd be happy with it to stay in this form. [...] Cheers ---Dave _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel