asm: simd context helper APIsimd

Sometimes it's useful to amortize calls to XSAVE/XRSTOR and the related
FPU/SIMD functions over a number of calls, because FPU restoration is
quite expensive. This adds a simple header for carrying out this pattern:

    simd_context_t simd_context;

    simd_get(&simd_context);
    while ((item = get_item_from_queue()) != NULL) {
        encrypt_item(item, &simd_context);
        simd_relax(&simd_context);
    }
    simd_put(&simd_context);

The relaxation step ensures that we don't trample over preemption, and
the get/put API should be a familiar paradigm in the kernel.

On the other end, code that actually wants to use SIMD instructions can
accept this as a parameter and check it via:

   void encrypt_item(struct item *item, simd_context_t *simd_context)
   {
       if (item->len > LARGE_FOR_SIMD && simd_use(simd_context))
           wild_simd_code(item);
       else
           boring_scalar_code(item);
   }

The actual XSAVE happens during simd_use (and only on the first time),
so that if the context is never actually used, no performance penalty is
hit.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Samuel Neves <sneves@dei.uc.pt>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: linux-arch@vger.kernel.org

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/sparc/include/asm/Kbuild b/arch/sparc/include/asm/Kbuild
index b82f64e28f55..d053d5bad630 100644
--- a/arch/sparc/include/asm/Kbuild
+++ b/arch/sparc/include/asm/Kbuild
@@ -19,5 +19,6 @@ generic-y += msi.h
 generic-y += preempt.h
 generic-y += rwsem.h
 generic-y += serial.h
+generic-y += simd.h
 generic-y += trace_clock.h
 generic-y += word-at-a-time.h