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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Wed, 22 Apr 2020 13:35:10 -0600
Message-ID: <CAHmME9ojNM1LUbbzD7uyZ3O5KWpT16urfsp=YMpx7LX0U=5bBA@mail.gmail.com>
Subject: Re: [PATCH crypto-stable] crypto: arch/lib - limit simd usage to
 PAGE_SIZE chunks
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc:     Ard Biesheuvel <ardb@kernel.org>,
        Eric Biggers <ebiggers@kernel.org>,
        linux-rt-users@vger.kernel.org,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Wed, Apr 22, 2020 at 5:28 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> On 2020-04-22 09:23:34 [+0200], Ard Biesheuvel wrote:
> > My memory is a bit fuzzy here. I remember talking to the linux-rt guys
> > about what delay is actually acceptable, which was a lot higher than I
> > had thought based on their initial reports about scheduling blackouts
> > on arm64 due to preemption remaining disabled for too long. I intended
> > to revisit this with more accurate bounds but then I apparently
> > forgot.
> >
> > So SIMD chacha20 and SIMD poly1305 both run in <5 cycles per bytes,
> > both on x86 and ARM. If we take 20 microseconds as a ballpark upper
> > bound for how long preemption may be disabled, that gives us ~4000
> > bytes of ChaCha20 or Poly1305 on a hypothetical 1 GHz core.
> >
> > So I think 4 KB is indeed a reasonable quantum of work here. Only
> > PAGE_SIZE is not necessarily equal to 4 KB on arm64, so we should use
> > SZ_4K instead.
> >
> > *However*, at the time, the report was triggered by the fact that we
> > were keeping SIMD enabled across calls into the scatterwalk API, which
> > may call kmalloc()/kfree() etc. There is no need for that anymore, now
> > that the FPU begin/end routines all have been optimized to restore the
> > userland SIMD state lazily.
>
> The 20usec sound reasonable. The other concern was memory allocation
> within the preempt-disable section. If this is no longer the case,
> perfect.

Cool, thanks for the confirmation. I'll get a v2 of this patch out the door.