From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DD0FEC433E0 for ; Tue, 9 Jun 2020 14:11:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BE6F42064C for ; Tue, 9 Jun 2020 14:11:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728952AbgFIOLw (ORCPT ); Tue, 9 Jun 2020 10:11:52 -0400 Received: from foss.arm.com ([217.140.110.172]:43026 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728338AbgFIOLw (ORCPT ); Tue, 9 Jun 2020 10:11:52 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 3FACE1FB; Tue, 9 Jun 2020 07:11:51 -0700 (PDT) Received: from arm.com (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 41CEE3F66F; Tue, 9 Jun 2020 07:11:50 -0700 (PDT) Date: Tue, 9 Jun 2020 15:11:42 +0100 From: Dave Martin To: Will Deacon Cc: Michael Kerrisk , linux-man@vger.kernel.org, linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Catalin Marinas Subject: Re: [PATCH v2 4/6] prctl.2: Add SVE prctls (arm64) Message-ID: <20200609140948.GA25945@arm.com> References: <1590614258-24728-1-git-send-email-Dave.Martin@arm.com> <1590614258-24728-5-git-send-email-Dave.Martin@arm.com> <20200609095734.GA25362@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200609095734.GA25362@willie-the-truck> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-man-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org On Tue, Jun 09, 2020 at 10:57:35AM +0100, Will Deacon wrote: > Hi Dave, > > On Wed, May 27, 2020 at 10:17:36PM +0100, Dave Martin wrote: > > Add documentation for the the PR_SVE_SET_VL and PR_SVE_GET_VL > > prctls added in Linux 4.15 for arm64. > > Looks really good to me, thanks. Just a few comments inline. > > > diff --git a/man2/prctl.2 b/man2/prctl.2 > > index cab9915..91df7c8 100644 > > --- a/man2/prctl.2 > > +++ b/man2/prctl.2 > > @@ -1291,6 +1291,148 @@ call failing with the error > > .BR ENXIO . > > For further details, see the kernel source file > > .IR Documentation/admin\-guide/kernel\-parameters.txt . > > +.\" prctl PR_SVE_SET_VL > > +.\" commit 2d2123bc7c7f843aa9db87720de159a049839862 > > +.\" linux-5.6/Documentation/arm64/sve.rst > > +.TP > > +.BR PR_SVE_SET_VL " (since Linux 4.15, only on arm64)" > > +Configure the thread's SVE vector length, > > +as specified by > > +.IR "(int) arg2" . > > +Arguments > > +.IR arg3 ", " arg4 " and " arg5 > > +are ignored. > > +.IP > > +The bits of > > +.I arg2 > > +corresponding to > > +.B PR_SVE_VL_LEN_MASK > > +must be set to the desired vector length in bytes. > > +This is interpreted as an upper bound: > > +the kernel will select the greatest available vector length > > +that does not exceed the value specified. > > +In particular, specifying > > +.B SVE_VL_MAX > > +(defined in > > +.I ) > > +for the > > +.B PR_SVE_VL_LEN_MASK > > +bits requests the maximum supported vector length. > > +.IP > > +In addition, > > +.I arg2 > > +must be set to one of the following combinations of flags: > > How about saying: > > In addition, the other bits of arg2 must be set according to the following > combinations of flags: > > Otherwise I find it a bit fiddly to read, because it's valid to have > flags of 0 and a non-zero length. 0 is listed, so I hoped that was clear enough. Maybe just write "must be one of the following values:"? 0 is a value, but I can see why you might be uneasy about 0 being described as a "combination of flags". > > +.RS > > +.TP > > +.B 0 > > +Perform the change immediately. > > +At the next > > +.BR execve (2) > > +in the thread, > > +the vector length will be reset to the value configured in > > +.IR /proc/sys/abi/sve_default_vector_length . > > (implementation note: does this mean that 'sve_default_vl' should be > an atomic_t, as it can be accessed concurrently? We probably need > {READ,WRITE}_ONCE() at the very least, as I'm not seeing any locks > that help us here...) Is this purely theoretical? Can you point to what could go wrong? While I doubt I thought about this very hard and I agree that you're right in principle, I think there are probably non-atomic sysctls and debugs files etc. all over the place. I didn't want to clutter the code unnecessarily. > > +.B PR_SVE_VL_INHERIT > > +Perform the change immediately. > > +Subsequent > > +.BR execve (2) > > +calls will preserve the new vector length. > > +.TP > > +.B PR_SVE_SET_VL_ONEXEC > > +Defer the change, so that it is performed at the next > > +.BR execve (2) > > +in the thread. > > +Further > > +.BR execve (2) > > +calls will reset the vector length to the value configured in > > +.IR /proc/sys/abi/sve_default_vector_length . > > +.TP > > +.B "PR_SVE_SET_VL_ONEXEC | PR_SVE_VL_INHERIT" > > +Defer the change, so that it is performed at the next > > +.BR execve (2) > > +in the thread. > > +Further > > +.BR execve (2) > > +calls will preserve the new vector length. > > +.RE > > +.IP > > +In all cases, > > +any previously pending deferred change is canceled. > > +.IP > > +The call fails with error > > +.B EINVAL > > +if SVE is not supported on the platform, if > > +.I arg2 > > +is unrecognized or invalid, or the value in the bits of > > +.I arg2 > > +corresponding to > > +.B PR_SVE_VL_LEN_MASK > > +is outside the range > > +.BR SVE_VL_MIN .. SVE_VL_MAX > > +or is not a multiple of 16. > > +.IP > > +On success, > > +a nonnegative value is returned that describes the > > +.I selected > > +configuration, > > If I'm reading the kernel code correctly, this is slightly weird, as > the returned value may contain the PR_SVE_VL_INHERIT flag but it will > never contain the PR_SVE_SET_VL_ONEXEC flag. Is that right? Yes, which is an oddity. I suppose we could fake that up actually by returning that flag if sve_vl and sve_vl_onexec are different, but we don't currently do this. > If so, maybe just say something like: > > On success, a nonnegative value is returned that describes the selected > configuration in the same way as PR_SVE_GET_VL. How does that help? PR_SVE_GET_VL doesn't fully clarify the oddity you call out anyway. Really, I preferred not to have people relying on this one way or the other. The only sensible reason for an _ONEXEC is because you've committed to calling execve(). On such a path, queryng the vector length isn't likely to be useful. Maybe I was optimistic. > > +which may differ from the current configuration if > > +.B PR_SVE_SET_VL_ONEXEC > > +was specified. > > +The value is encoded in the same way as the return value of > > +.BR PR_SVE_GET_VL . > > +.IP > > +The configuration (including any pending deferred change) > > +is inherited across > > +.BR fork (2) > > +and > > +.BR clone (2). > > +.IP > > +.B Warning: > > +Because the compiler or run-time environment > > +may be using SVE, using this call without the > > +.B PR_SVE_SET_VL_ONEXEC > > +flag may crash the calling process. > > +The conditions for using it safely are complex and system-dependent. > > +Don't use it unless you really know what you are doing. > > +.IP > > +For more information, see the kernel source file > > +.I Documentation/arm64/sve.rst > > +.\"commit b693d0b372afb39432e1c49ad7b3454855bc6bed > > +(or > > +.I Documentation/arm64/sve.txt > > +before Linux 5.3). > > I think I'd drop the kernel reference here, as it feels like we're saying > "only do this if you know what you're doing" on one hand, but then "if you > don't know what you're doing, see this other documentation" on the other. Well, the docmuentation doesn't answer those questions either. I could just swap the warning and the cross-reference, so that the cross-reference doesn't seem to follow on from "knowing what you're doing"? Cheers ---Dave