linux-kernel - RE: [PATCH v2 04/19] crypto: x86/sha

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Date:   Fri, 14 Oct 2022 11:01:12 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Jason A. Donenfeld'" <Jason@...c4.com>,
        Robert Elliott <elliott@....com>
CC:     "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "ap420073@...il.com" <ap420073@...il.com>,
        "ardb@...nel.org" <ardb@...nel.org>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 04/19] crypto: x86/sha - limit FPU preemption

From: Jason A. Donenfeld
> Sent: 13 October 2022 01:42
...
> > diff --git a/arch/x86/crypto/sha1_ssse3_glue.c b/arch/x86/crypto/sha1_ssse3_glue.c
> > index 44340a1139e0..a9f5779b41ca 100644
> > --- a/arch/x86/crypto/sha1_ssse3_glue.c
> > +++ b/arch/x86/crypto/sha1_ssse3_glue.c
> > @@ -26,6 +26,8 @@
> >  #include <crypto/sha1_base.h>
> >  #include <asm/simd.h>
> >
> > +#define FPU_BYTES 4096U /* avoid kernel_fpu_begin/end scheduler/rcu stalls */
> 
> Declare this inside the function it's used as an untyped enum, and give
> it a better name, like BYTES_PER_FPU.

Isn't 'bytes' the wrong unit anyway?
At least it ought to be 'clocks' so it can be divided by the
(approximate) 'clocks per byte' of the algorithm.

Something like a crc is likely to be far faster than AES.

Clearly the actual required units are microseconds.
But depending on the actual cpu frequency is a bit hard.
And people running faster cpu may want lower latency anyway.
So a typical slow cpu frequency is probably ok.

The actual architecture dependant constant really ought
to be defined with kernel_fpu_begin().

	David

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