linux-kernel - Re: [PATCH v5 4/5] powerpc/vdso: Wire up getrandom() vDSO implementation on VDSO32

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Message-ID: <ZtnYqZI-nrsNslwy@zx2c4.com>
Date: Thu, 5 Sep 2024 18:13:29 +0200
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: Christophe Leroy <christophe.leroy@...roup.eu>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Naveen N Rao <naveen@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-kselftest@...r.kernel.org,
	llvm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org,
	Adhemerval Zanella <adhemerval.zanella@...aro.org>,
	Xi Ruoyao <xry111@...111.site>
Subject: Re: [PATCH v5 4/5] powerpc/vdso: Wire up getrandom() vDSO
 implementation on VDSO32

> +/*
> + * The macro sets two stack frames, one for the caller and one for the callee
> + * because there are no requirement for the caller to set a stack frame when
> + * calling VDSO so it may have omitted to set one, especially on PPC64
> + */
> +
> +.macro cvdso_call funct
> +  .cfi_startproc
> +	PPC_STLU	r1, -PPC_MIN_STKFRM(r1)
> +  .cfi_adjust_cfa_offset PPC_MIN_STKFRM
> +	mflr		r0
> +	PPC_STLU	r1, -PPC_MIN_STKFRM(r1)
> +  .cfi_adjust_cfa_offset PPC_MIN_STKFRM
> +	PPC_STL		r0, PPC_MIN_STKFRM + PPC_LR_STKOFF(r1)
> +  .cfi_rel_offset lr, PPC_MIN_STKFRM + PPC_LR_STKOFF
> +	get_datapage	r8
> +	addi		r8, r8, VDSO_RNG_DATA_OFFSET
> +	bl		CFUNC(DOTSYM(\funct))
> +	PPC_LL		r0, PPC_MIN_STKFRM + PPC_LR_STKOFF(r1)
> +	cmpwi		r3, 0
> +	mtlr		r0
> +	addi		r1, r1, 2 * PPC_MIN_STKFRM
> +  .cfi_restore lr
> +  .cfi_def_cfa_offset 0
> +	crclr		so
> +	bgelr+
> +	crset		so
> +	neg		r3, r3
> +	blr
> +  .cfi_endproc
> +.endm

You wrote in an earlier email that this worked with time namespaces, but
in my testing that doesn't seem to be the case.

>From my test harness [1]:

Normal single thread
   vdso: 25000000 times in 12.494133131 seconds
   libc: 25000000 times in 69.594625188 seconds
syscall: 25000000 times in 67.349243972 seconds
Time namespace single thread
   vdso: 25000000 times in 71.673057436 seconds
   libc: 25000000 times in 71.712774121 seconds
syscall: 25000000 times in 66.902318080 seconds

I'm seeing this on ppc, ppc64, and ppc64le.

Can you figure out what's going on and send a fix, which I'll squash
into this commit?

Jason

[1] https://git.zx2c4.com/linux-rng/commit/?h=jd/vdso-test-harness