Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 23 Sep 2020 12:19:53 +0200
From:   peterz@infradead.org
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Paul McKenney <paulmck@kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>,
        Russell King <linux@armlinux.org.uk>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Chris Zankel <chris@zankel.net>,
        Max Filippov <jcmvbkbc@gmail.com>,
        linux-xtensa@linux-xtensa.org,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        intel-gfx <intel-gfx@lists.freedesktop.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Ard Biesheuvel <ardb@kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Vineet Gupta <vgupta@synopsys.com>,
        "open list\:SYNOPSYS ARC ARCHITECTURE" 
        <linux-snps-arc@lists.infradead.org>,
        Arnd Bergmann <arnd@arndb.de>, Guo Ren <guoren@kernel.org>,
        linux-csky@vger.kernel.org, Michal Simek <monstr@monstr.eu>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        linux-mips@vger.kernel.org, Nick Hu <nickhu@andestech.com>,
        Greentime Hu <green.hu@gmail.com>,
        Vincent Chen <deanbo422@gmail.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        "David S. Miller" <davem@davemloft.net>,
        linux-sparc <sparclinux@vger.kernel.org>
Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of
 kmap_atomic & friends
Message-ID: <20200923101953.GT2674@hirez.programming.kicks-ass.net>
References: <20200919091751.011116649@linutronix.de>
 <CAHk-=wiYGyrFRbA1cc71D2-nc5U9LM9jUJesXGqpPnB7E4X1YQ@mail.gmail.com>
 <87mu1lc5mp.fsf@nanos.tec.linutronix.de>
 <87k0wode9a.fsf@nanos.tec.linutronix.de>
 <CAHk-=wgbmwsTOKs23Z=71EBTrULoeaH2U3TNqT2atHEWvkBKdw@mail.gmail.com>
 <87eemwcpnq.fsf@nanos.tec.linutronix.de>
 <CAHk-=wgF-upZVpqJWK=TK7MS9H-Rp1ZxGfOG+dDW=JThtxAzVQ@mail.gmail.com>
 <87a6xjd1dw.fsf@nanos.tec.linutronix.de>
 <CAHk-=wjhxzx3KHHOMvdDj3Aw-_Mk5eRiNTUBB=tFf=vTkw1FeA@mail.gmail.com>
 <87sgbbaq0y.fsf@nanos.tec.linutronix.de>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <87sgbbaq0y.fsf@nanos.tec.linutronix.de>
Precedence: bulk

On Mon, Sep 21, 2020 at 09:27:57PM +0200, Thomas Gleixner wrote:
> Alternatively this could of course be solved with per CPU page tables
> which will come around some day anyway I fear.

Previously (with PTI) we looked at making the entire kernel map per-CPU,
and that takes a 2K copy on switch_mm() (or more general, the user part
of whatever the top level directory is for architectures that have a
shared kernel/user page-table setup in the first place).

The idea was having a fixed per-cpu kernel page-table, share a bunch of
(kernel) page-tables between all CPUs and then copy in the user part on
switch.

I've forgotten what the plan was for ASID/PCID in that scheme.

For x86_64 we've been fearing the performance of that 2k copy, but I
don't think we've ever actually bit the bullet and implemented it to see
how bad it really is.