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Date:   Tue, 1 Feb 2022 16:20:40 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fw@deneb.enyo.de>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        paulmck <paulmck@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api <linux-api@vger.kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        David Laight <David.Laight@ACULAB.COM>,
        carlos <carlos@redhat.com>, Peter Oskolkov <posk@posk.io>
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Subject: Re: [RFC PATCH 2/3] rseq: extend struct rseq with per thread group
 vcpu id
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Thread-Topic: rseq: extend struct rseq with per thread group vcpu id
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----- On Feb 1, 2022, at 3:32 PM, Florian Weimer fw@deneb.enyo.de wrote:
[...]
>=20
>>> Is the switch really useful?  I suspect it's faster to just write as
>>> much as possible all the time.  The switch should be well-predictable
>>> if running uniform userspace, but still =E2=80=A6
>>
>> The switch ensures the kernel don't try to write to a memory area beyond
>> the rseq size which has been registered by user-space. So it seems to be
>> useful to ensure we don't corrupt user-space memory. Or am I missing you=
r
>> point ?
>=20
> Due to the alignment, I think you'd only ever see 32 and 64 bytes for
> now?

Yes, but I would expect the rseq registration arguments to have a rseq_len
of offsetofend(struct rseq, tg_vcpu_id) when userspace wants the tg_vcpu_id
feature to be supported (but not the following features).

Then, as we append additional features as follow-up fields, those
eventually become requested by glibc by increasing the requested size.

Then it's kind of weird to receive a registration size which is not
aligned on 32-byte, but then use internal knowledge of the structure
alignment in the kernel code to write beyond the requested size. And all
this in a case where we are returning to user-space after a preemption,
so I don't expect this extra switch/case to cause significant overhead.

>=20
> I'd appreciate if you could put the maximm supported size and possibly
> the alignment in the auxiliary vector, so that we don't have to rseq
> system calls in a loop on process startup.

Yes, it's a good idea. I'm not too familiar with the auxiliary vector.
Are we talking about the kernel's

fs/binfmt_elf.c:fill_auxv_note()

?

Thanks,

Mathieu

--=20
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com