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From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 13 Apr 2021 18:57:08 +0200
Message-ID: <CANn89iJi=RY5HE6+TDvNv0HPEuedtsYHkEZSoEb45EO=tQM2tw@mail.gmail.com>
Subject: Re: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs()
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Eric Dumazet <eric.dumazet@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        paulmck <paulmck@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
        Arjun Roy <arjunroy@google.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Tue, Apr 13, 2021 at 6:54 PM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> ----- On Apr 13, 2021, at 12:22 PM, Eric Dumazet eric.dumazet@gmail.com wrote:
>
> > From: Eric Dumazet <edumazet@google.com>
> >
> > Commit ec9c82e03a74 ("rseq: uapi: Declare rseq_cs field as union,
> > update includes") added regressions for our servers.
> >
> > Using copy_from_user() and clear_user() for 64bit values
> > is suboptimal.
> >
> > We can use faster put_user() and get_user().
> >
> > 32bit arches can be changed to use the ptr32 field,
> > since the padding field must always be zero.
> >
> > v2: added ideas from Peter and Mathieu about making this
> >    generic, since my initial patch was only dealing with
> >    64bit arches.
>
> Ah, now I remember the reason why reading and clearing the entire 64-bit
> is important: it's because we don't want to allow user-space processes to
> use this change in behavior to figure out whether they are running on a
> 32-bit or in a 32-bit compat mode on a 64-bit kernel.
>
> So although I'm fine with making 64-bit kernels faster, we'll want to keep
> updating the entire 64-bit ptr field on 32-bit kernels as well.
>
> Thanks,
>

So... back to V1 then ?