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From:   Andy Lutomirski <luto@amacapital.net>
Mime-Version: 1.0 (1.0)
Subject: Re: [PATCH v9 00/17] Enable FSGSBASE instructions
Date:   Tue, 21 Apr 2020 09:49:11 -0700
Message-Id: <BB51CFEA-A635-4664-821C-B57094AE95F0@amacapital.net>
References: <20200421160622.GJ1809@sasha-vm>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Andi Kleen <ak@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        "Metzger, Markus T" <markus.t.metzger@intel.com>,
        "hpa@zytor.com" <hpa@zytor.com>, "bp@alien8.de" <bp@alien8.de>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        "Luck, Tony" <tony.luck@intel.com>,
        Pedro Alves <palves@redhat.com>,
        Simon Marchi <simark@simark.ca>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
In-Reply-To: <20200421160622.GJ1809@sasha-vm>
To:     Sasha Levin <sashal@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Apr 21, 2020, at 9:06 AM, Sasha Levin <sashal@kernel.org> wrote:
>=20
> =EF=BB=BFOn Mon, Apr 20, 2020 at 07:14:46PM +0200, Thomas Gleixner wrote:
>> Andi Kleen <ak@linux.intel.com> writes:
>>>> the *gdb developers* don't care.  But gdb isn't exactly a good example
>>>> of a piece of software that tries to work correctly when dealing with
>>>> unusual software.  Maybe other things like rr will care more.  It
>>>=20
>>> rr is used to replay modern software, and modern software
>>> doesn't care about selectors, thus rr doesn't care either.
>>>=20
>>> Please stop the FUD.
>>=20
>> There is absolutely no FUD. Being careful about not breaking existing
>> user space is a legitimate request.
>>=20
>> It's up to those who change the ABI to prove that it does not matter and
>> not up to the maintainers to figure it out.
>=20
> I think that this is a difficult ask; "prove that god doesn't exist".
>=20
> Andi's point is that there is no known user it breaks, and the Intel
> folks did some digging into potential users who might be affected by
> this, including 'rr' brought up by Andy, and concluded that there won't
> be breakage as a result of this patchset:
>=20
>    https://mail.mozilla.org/pipermail/rr-dev/2018-March/000616.html
>=20
> Sure, if you poke at it you could see a behavior change, but is there
> an actual user that will be affected by it? I suspect not.
>=20
>> This sits in limbo for months now just because Intel doesn't get it's
>> homework done.
>>=20
>> Stop making false accusations and provide factual information instead.
>=20
> If there's no known user that will be broken here, can we consider
> merging this to be disabled by default and let distros try it out? This
> will let us find these users while providing an easy way to work around
> the problem.

No.  Once it=E2=80=99s merged, people will write user code using the ABI, an=
d that means we need to get the ABI right.

The very early versions had severely problematic ABIs. The new ones are prob=
ably okay except for, maybe, ptrace.  If we had merged the old ones, then we=
 might have gotten stuck with the old, problematic ABI.

>=20
> --=20
> Thanks,
> Sasha