linux-kernel - Re: [RFC PATCH 01/20] asm-generic/mmiowb: Add generic implementation of mmiowb() tracking

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20190226183335.GN2861@worktop.programming.kicks-ass.net>
Date:   Tue, 26 Feb 2019 19:33:35 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnd Bergmann <arnd@...db.de>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        Ingo Molnar <mingo@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, Tony Luck <tony.luck@...el.com>
Subject: Re: [RFC PATCH 01/20] asm-generic/mmiowb: Add generic implementation
 of mmiowb() tracking

On Tue, Feb 26, 2019 at 06:26:24PM +0000, Will Deacon wrote:
> On Fri, Feb 22, 2019 at 01:49:32PM -0800, Linus Torvalds wrote:
> So I *am* using __this_cpu_xchg() here, which means the architecture can
> get away with plain old loads and stores (which is what RISC-V does, for
> example), but I see that's not the case on e.g. x86 so I'll rework using
> read() and write() because it doesn't hurt.

Right, so the problem on x86 is that XCHG has an implicit LOCK prefix,
so there no !atomic variant. So even the cpu-local xchg gets the LOCK
prefix penalty, even though all we really wanted is a single
instruction.

Arguably we could fix that for __this_cpu_xchg(), which isn't IRQ-safe.