From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5DE5215C133 for ; Sat, 7 Sep 2024 14:36:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725719764; cv=none; b=C7NVmqnMI8PX1g42zea/MPCxITdRu58z6yrjQA9vjZUTVrUvktAZmCiMwXadBg1xbhylKoNMsYmLCEiskxT0Wwmi8jArDrA0CY+VLzyMCww7OxQzfuh0XCWninIF1FEWb0b+Ysb4+s1Qs5ftTAb+3Qg6lXI3oqCkjIvkU0dJvno= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725719764; c=relaxed/simple; bh=pRYD/cbq9Oa8VPa231IqFnxaPN1Vce3vzUD8u9v7QgQ=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=LOG3S2XBvM3GJGVFu7MisSZVIf4qQ+cw6NmouaM7ZQ8I+1lLniW4+XIUkzH0NmKRa6bcEb1+qOOE3FECjedqRzIbwTHlXDGio4ZzvPwWcNuvPm0rIRxUbHS+x/DFq4UcKWKXsLJTUdxYhmOg9i1x4yeuuvLNFjqj1En1I4PpOhs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b=cgY7SHp1; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="cgY7SHp1" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 18471C4CECA; Sat, 7 Sep 2024 14:36:03 +0000 (UTC) Authentication-Results: smtp.kernel.org; dkim=pass (1024-bit key) header.d=zx2c4.com header.i=@zx2c4.com header.b="cgY7SHp1" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=zx2c4.com; s=20210105; t=1725719760; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=zaEo8gilrmxogMmzsNMScJpOwzlO2ivQy8zmDfqEv4A=; b=cgY7SHp1xO5oPO5RsEP47PF73VB+d0FY23/q67jdWND6uXjRx0pjw091wozLwxTM67sfgo G+pFQr56sf6mr4hw5zxBn2Th+pLJXT8J493ic5hi2mZo50MOpx5fg7zBvvYUvBgfXIgM7x wEtG6GPUnraNdvCfRfTKwL8wQRpVLvM= Received: by mail.zx2c4.com (ZX2C4 Mail Server) with ESMTPSA id 1d2759ad (TLSv1.3:TLS_AES_256_GCM_SHA384:256:NO); Sat, 7 Sep 2024 14:36:00 +0000 (UTC) Date: Sat, 7 Sep 2024 16:35:58 +0200 From: "Jason A. Donenfeld" To: Christophe Leroy Cc: linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, Michael Ellerman , Nicholas Piggin , Naveen N Rao , Vincenzo Frascino , Andrei Vagin Subject: Re: [PATCH 2/2] Fixup for 3279be36b671 ("powerpc/vdso: Wire up getrandom() vDSO implementation on VDSO32") Message-ID: References: <700dbf296d02e32376329774be35cfbead08041d.1725611321.git.christophe.leroy@csgroup.eu> <795db5f1-c266-4fb3-a51b-c2b3745d334b@csgroup.eu> <8d0a8d03-95b3-40a8-85cd-5c2e6f92eb6b@csgroup.eu> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, Sep 06, 2024 at 08:54:49PM +0200, Jason A. Donenfeld wrote: > On Fri, Sep 06, 2024 at 05:14:43PM +0200, Christophe Leroy wrote: > > > > > > Le 06/09/2024 à 16:46, Jason A. Donenfeld a écrit : > > > On Fri, Sep 06, 2024 at 04:26:32PM +0200, Christophe Leroy wrote: > > > > > >> On the long run I wonder if we should try to find a more generic > > >> solution for getrandom instead of requiring each architecture to handle > > >> it. On gettimeofday the selection of the right page is embeded in the > > >> generic part, see for instance : > > >> > > >> static __maybe_unused __kernel_old_time_t > > >> __cvdso_time_data(const struct vdso_data *vd, __kernel_old_time_t *time) > > >> { > > >> __kernel_old_time_t t; > > >> > > >> if (IS_ENABLED(CONFIG_TIME_NS) && > > >> vd->clock_mode == VDSO_CLOCKMODE_TIMENS) > > >> vd = __arch_get_timens_vdso_data(vd); > > >> > > >> t = READ_ONCE(vd[CS_HRES_COARSE].basetime[CLOCK_REALTIME].sec); > > >> > > >> if (time) > > >> *time = t; > > >> > > >> return t; > > >> } > > >> > > >> and powerpc just provides: > > >> > > >> static __always_inline > > >> const struct vdso_data *__arch_get_timens_vdso_data(const struct > > >> vdso_data *vd) > > >> { > > >> return (void *)vd + (1U << CONFIG_PAGE_SHIFT); > > >> } > > > > > > It's tempting, but maybe a bit tricky. LoongArch, for example, doesn't > > > have this problem at all, because the layout of their vvars doesn't > > > require it. So the vd->clock_mode access is unnecessary. > > > > > >> Or another solution could be to put random data in a third page that is > > >> always at the same place regardless of timens ? > > > > > > Maybe that's the easier way, yea. Potentially wasteful, though. > > > > > > > Indeed I just looked at Loongarch and that's exactly what they do: they > > have a third page after the two pages dedicated to TIME for arch > > specific data, and they have added getrandom data there. > > > > The third page is common to every process so it won't waste more than a > > few bytes. It doesn't worry me even on the older boards that only have > > 32 Mbytes of RAM. > > > > So yes, I may have a look at that in the future, what we have at the > > moment is good enough to move forward. > > My x86 code is kind of icky for this: > > static __always_inline const struct vdso_rng_data *__arch_get_vdso_rng_data(void) > { > if (IS_ENABLED(CONFIG_TIME_NS) && __vdso_data->clock_mode == VDSO_CLOCKMODE_TIMENS) > return (void *)&__vdso_rng_data + ((void *)&__timens_vdso_data - (void *)&__vdso_data); > return &__vdso_rng_data; > } > > Doing the subtraction like that means that this is more clearly correct. > But it also makes the compiler insert two jumps for the branch, and then > reads the addresses of those variables and such. > > If I change it to: > > static __always_inline const struct vdso_rng_data *__arch_get_vdso_rng_data(void) > { > if (IS_ENABLED(CONFIG_TIME_NS) && __vdso_data->clock_mode == VDSO_CLOCKMODE_TIMENS) > return (void *)&__vdso_rng_data + (3UL << CONFIG_PAGE_SHIFT); > return &__vdso_rng_data; > } > > Then there's a much nicer single `cmov` with no branching. > > But if I want to do that for real, I'll have to figure out what set of > nice compile-time constants I can use. I haven't looked into this yet. https://lore.kernel.org/all/20240906190655.2777023-1-Jason@zx2c4.com/