From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Eric Dumazet <edumazet@google.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>
Subject: Re: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs()
Date: Tue, 13 Apr 2021 13:06:16 -0400 (EDT) [thread overview]
Message-ID: <373117462.72486.1618333576943.JavaMail.zimbra@efficios.com> (raw)
In-Reply-To: <CANn89iJi=RY5HE6+TDvNv0HPEuedtsYHkEZSoEb45EO=tQM2tw@mail.gmail.com>
----- On Apr 13, 2021, at 12:57 PM, Eric Dumazet edumazet@google.com wrote:
> 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 ?
In terms of behavior, yes. And it's probably the "easy" fix, but I hate that
it adds lots of preprocessor ifdefs into the rseq code.
But this would require auditing get_user()/put_user() for each architecture
supported by rseq to ensure they support 8-byte load/store. And it would become
an added burden on architecture maintainers wishing to add rseq support for their
architecture.
One alternative would be to implement rseq_get_user_u64 and rseq_put_user_u64
wrappers as static functions within rseq.c to hide the preprocessor ifdeffery
from the higher-level code. I try very hard to avoid mixing preprocessor ifdefs
with C code logic whenever I can.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
prev parent reply other threads:[~2021-04-13 17:06 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-13 16:22 [PATCH v2 0/3] rseq: minor optimizations Eric Dumazet
2021-04-13 16:22 ` [PATCH v2 1/3] rseq: optimize rseq_update_cpu_id() Eric Dumazet
2021-04-13 16:22 ` [PATCH v2 2/3] rseq: remove redundant access_ok() Eric Dumazet
2021-04-13 16:22 ` [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs() Eric Dumazet
2021-04-13 16:54 ` Mathieu Desnoyers
2021-04-13 16:57 ` Eric Dumazet
2021-04-13 17:01 ` Eric Dumazet
2021-04-13 17:07 ` Eric Dumazet
2021-04-13 17:20 ` Mathieu Desnoyers
2021-04-13 17:33 ` Eric Dumazet
2021-04-13 18:00 ` Mathieu Desnoyers
2021-04-13 18:22 ` Eric Dumazet
2021-04-13 18:35 ` Arjun Roy
2021-04-13 21:19 ` David Laight
2021-04-13 22:03 ` Arjun Roy
2021-04-14 7:55 ` David Laight
2021-04-14 16:00 ` Eric Dumazet
2021-04-14 16:08 ` David Laight
2021-04-14 16:10 ` Eric Dumazet
2021-04-14 17:15 ` Arjun Roy
2021-04-14 17:35 ` Eric Dumazet
2021-04-14 20:15 ` Arjun Roy
2021-04-14 20:25 ` Eric Dumazet
2021-04-14 20:35 ` Arjun Roy
2021-04-13 19:13 ` Mathieu Desnoyers
2021-04-13 17:06 ` Mathieu Desnoyers [this message]
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