linux-kernel - Re: [PATCH crypto-stable] crypto: arch/lib - limit simd usage to PAGE

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Message-ID: <CAHmME9qfrHVQ+-4HjqCO2TaGF6DNTHmS1max1KcVaP5_QjUDRQ@mail.gmail.com>
Date:   Mon, 20 Apr 2020 22:02:31 -0600
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ebiggers@...gle.com" <ebiggers@...gle.com>,
        "ardb@...nel.org" <ardb@...nel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH crypto-stable] crypto: arch/lib - limit simd usage to
 PAGE_SIZE chunks

On Mon, Apr 20, 2020 at 2:32 AM David Laight <David.Laight@...lab.com> wrote:
> Maybe kernel_fp_begin() should be passed the address of somewhere
> the address of an fpu save area buffer can be written to.
> Then the pre-emption code can allocate the buffer and save the
> state into it.
>
> However that doesn't solve the problem for non-preemptive kernels.
> The may need a cond_resched() in the loop if it might take 1ms (or so).
>
> kernel_fpu_begin() ought also be passed a parameter saying which
> fpu features are required, and return which are allocated.
> On x86 this could be used to check for AVX512 (etc) which may be
> available in an ISR unless it interrupted inside a kernel_fpu_begin()
> section (etc).
> It would also allow optimisations if only 1 or 2 fpu registers are
> needed (eg for some of the crypto functions) rather than the whole
> fpu register set.

There might be ways to improve lots of FPU things, indeed. This patch
here is just a patch to Herbert's branch in order to make uniform
usage of our existing solution for this, fixing the existing bug. I
wouldn't mind seeing more involved and better solutions in a patchset
for crypto-next.

Will follow up with your suggestion in a different thread, so as not
to block this one.