linux-kernel - Re: [PATCH 7/7] s390/vdso: Wire up getrandom() vdso implementation

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Message-ID: <ZuRD58DrEzzcXKZg@zx2c4.com>
Date: Fri, 13 Sep 2024 15:53:43 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: Alexander Gordeev <agordeev@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Harald Freudenberger <freude@...ux.ibm.com>,
	Stefan Liebler <stli@...ux.ibm.com>, linux-kernel@...r.kernel.org,
	linux-crypto@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH 7/7] s390/vdso: Wire up getrandom() vdso implementation

On Fri, Sep 13, 2024 at 03:05:43PM +0200, Heiko Carstens wrote:
> Provide the s390 specific vdso getrandom() architecture backend.
> 
> _vdso_rng_data required data is placed within the _vdso_data vvar page, by
> using a hardcoded offset larger than vdso_data.
> 
> As required the chacha20 implementation does not write to the stack.
> 
> The implementation follows more or less the arm64 implementations and
> makes use of vector instructions. It has a fallback to the getrandom()
> system call for machines where the vector facility is not
> installed.
> The check if the vector facility is installed, as well as an
> optimization for machines with the vector-enhancements facility 2,
> is implemented with alternatives, avoiding runtime checks.
> 
> Note that __kernel_getrandom() is implemented without the vdso user wrapper
> which would setup a stack frame for odd cases (aka very old glibc variants)
> where the caller has not done that. All callers of __kernel_getrandom() are
> required to setup a stack frame, like the C ABI requires it.
> 
> The vdso testcases vdso_test_getrandom and vdso_test_chacha pass.

I'd be curious to see the results of ./vdso_test_getrandom bench-single
and such.

Jason