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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [patch V4 part 3 01/29] x86/traps: Mark fixup_bad_iret() noinstr
Date:   Wed, 13 May 2020 18:35:40 -0700
Message-Id: <DBE4DB85-973F-4E16-8574-B93F9E4A51BC@amacapital.net>
References: <807929267.20601.1589416908153.JavaMail.zimbra@efficios.com>
Cc:     rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        x86 <x86@kernel.org>, paulmck <paulmck@kernel.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <JGross@suse.com>,
        Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>
In-Reply-To: <807929267.20601.1589416908153.JavaMail.zimbra@efficios.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On May 13, 2020, at 5:41 PM, Mathieu Desnoyers <mathieu.desnoyers@efficios=
.com> wrote:
>=20
> =EF=BB=BF----- On May 12, 2020, at 9:51 PM, rostedt rostedt@goodmis.org wr=
ote:
>=20
>>> On Fri, 8 May 2020 17:39:00 -0700
>>> Andy Lutomirski <luto@kernel.org> wrote:
>>>=20
>>> On Tue, May 5, 2020 at 7:15 AM Thomas Gleixner <tglx@linutronix.de> wrot=
e:
>>>>=20
>>>> This is called from deep entry ASM in a situation where instrumentation=

>>>> will cause more harm than providing useful information.
>>>>=20
>>>=20
>>> Acked-by: Andy Lutomirski <luto@kernel.org>
>>>=20
>>> Maybe add to changelog:
>>>=20
>>> Switch from memmove() to memcpy() because memmove() can't be called
>>> from noinstr code.
>>=20
>> Yes please, because I was about to say that there was changes that
>> didn't seem to fit the change log.
>>=20
>> I would also add a comment in the code saying that we need the temp
>> variable to use memcpy as memmove can't be used in noinstr code.
>=20
> Looking at an updated version of the tree, I see the acked-by from Andy,
> but not comment about switching from memmove to memcpy.
>=20
> Also, I notice a significant undocumented change in this patch: it changes=

> a this_cpu_read() (which presumes preemption is enabled) to a __this_cpu_r=
ead().
>=20
> So the 100$ question: is preemption enabled or not in fixup_bad_iret() ? A=
nd of
> course that change should be documented in the commit message.
>=20

IRQs are off, and, if they were on, the lack of a warning from the percpu ac=
cess would be the least of our concerns here.

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