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McKenney" , Boqun Feng , Andy Lutomirski , Dave Watson , linux-kernel , linux-api , Paul Turner , Andrew Morton , Russell King , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andrew Hunter , Andi Kleen , Chris Lameter , Ben Maurer , rostedt , Josh Triplett , Catalin Marinas , Will Deacon , Michael Kerrisk Message-ID: <1580648199.9463.1523563167045.JavaMail.zimbra@efficios.com> In-Reply-To: References: <20180412192800.15708-1-mathieu.desnoyers@efficios.com> <20180412192800.15708-13-mathieu.desnoyers@efficios.com> Subject: Re: [RFC PATCH for 4.18 12/23] cpu_opv: Provide cpu_opv system call (v7) MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.142.138] X-Mailer: Zimbra 8.8.7_GA_1964 (ZimbraWebClient - FF52 (Linux)/8.8.7_GA_1964) Thread-Topic: cpu_opv: Provide cpu_opv system call (v7) Thread-Index: waubI5rPDESSVglebr9CwnpMfIO0/w== Sender: linux-api-owner@vger.kernel.org X-Mailing-List: linux-api@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: ----- On Apr 12, 2018, at 3:43 PM, Linus Torvalds torvalds@linux-foundation.org wrote: > On Thu, Apr 12, 2018 at 12:27 PM, Mathieu Desnoyers > wrote: >> The cpu_opv system call executes a vector of operations on behalf of >> user-space on a specific CPU with preemption disabled. It is inspired >> by readv() and writev() system calls which take a "struct iovec" >> array as argument. > > Do we really want the page pinning? > > This whole cpu_opv thing is the most questionable part of the series, > and the page pinning is the most questionable part of cpu_opv for me. What are your concerns about page pinning ? Do you have an alternative approach in mind ? > Can we plan on merging just the plain rseq parts *without* this all > first, and then see the cpu_opv thing as a "maybe future expansion" > part. The main problem with the incremental approach is that it won't deal with remote CPU data accesses, and won't deal with cpu hotplug in non-racy ways. For *some* of the use-cases, the other issues solved by cpu_opv can be worked-around in user-space, at the cost of making the userspace code a mess, and in many cases slower than if we can rely on cpu_opv for the fallback. All the rseq test-cases depend on cpu_opv as they stand now. Without cpu_opv to handle the corner-cases, things become much more messy on the user-space side. Thanks, Mathieu > > I think that would make Andy happier too. > > Linus -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com