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Subject: Re: [PATCH] x86/mm/fault: Streamline the fault error_code decoder some more
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20181206164710.GA119529@gmail.com>
Date:   Thu, 6 Dec 2018 09:05:11 -0800
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Rik van Riel <riel@surriel.com>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>
Content-Transfer-Encoding: quoted-printable
Message-Id: <EF7E7857-B4DF-4C14-85F5-114B03D5295D@amacapital.net>
References: <20181205163624.1842-1-sean.j.christopherson@intel.com> <20181206073409.GA64887@gmail.com> <20181206155336.GC31263@linux.intel.com> <C1F7477E-70B1-4F26-9457-EA6D825501A6@amacapital.net> <20181206164710.GA119529@gmail.com>
To:     Ingo Molnar <mingo@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Dec 6, 2018, at 8:47 AM, Ingo Molnar <mingo@kernel.org> wrote:
>=20
>=20
> * Andy Lutomirski <luto@amacapital.net> wrote:
>=20
>>> vs. (with SGX added as 'G' for testing purposes)
>>>=20
>>> [    0.158849] #PF error code(0001):  +P !W !U !S !I !K !G
>>> [    0.159292] #PF error code(0003):  +P +W !U !S !I !K !G
>>> [    0.159742] #PF error code(0007):  +P +W +U !S !I !K !G
>>> [    0.160190] #PF error code(0025):  +P !W +U !S !I +K !G
>>> [    0.160638] #PF error code(0002):  !P +W !U !S !I !K !G
>>> [    0.161087] #PF error code(0004):  !P !W +U !S !I !K !G
>>> [    0.161538] #PF error code(0006):  !P +W +U !S !I !K !G
>>> [    0.161992] #PF error code(0014):  !P !W +U !S +I !K !G
>>> [    0.162450] #PF error code(0011):  +P !W !U !S +I !K !G
>>> [    0.162667] #PF error code(8001):  +P !W !U !S !I !K +G
>>> [    0.162667] #PF error code(8003):  +P +W !U !S !I !K +G
>>> [    0.162667] #PF error code(8007):  +P +W +U !S !I !K +G
>>> [    0.162667] #PF error code(8025):  +P !W +U !S !I +K +G
>>> [    0.162667] #PF error code(8002):  !P +W !U !S !I !K +G
>>> [    0.162667] #PF error code(8004):  !P !W +U !S !I !K +G
>>> [    0.162667] #PF error code(8006):  !P +W +U !S !I !K +G
>>> [    0.162667] #PF error code(8014):  !P !W +U !S +I !K +G
>>> [    0.162667] #PF error code(8011):  +P !W !U !S +I !K +G
>>> [    0.162667] #PF error code(0000):  !P !W !U !S !I !K !G
>>>=20
>>=20
>> Please don=E2=80=99t. The whole reason I added the decoding was to make i=
t easy=20
>> to read without a cheat sheet. This is incomprehensible without=20
>> reference to the code, and I=E2=80=99m familiar with it to begin with.
>=20
> Dunno, I can deduct the meaning from the above abbreviations without a=20
> cheat sheet and I'm sure you'll be able to too from now on. All the=20
> letters are very obvious references - to me at least, and brevity and=20
> predictable, fixed-length output matters.
>=20
>> How about:
>>=20
>> #PF error code: 0001 [PROT read kernel]
>>=20
>> #PF error code: 0001 [PROT WRITE kernel]
>>=20
>> #PF error code: 0001 [PROT read kernel]
>>=20
>> #PF error code: 8011 [PROT INSTR kernel SGX]
>>=20
>> This has no noise from unset bits except that we add lowercase =E2=80=9Cr=
ead=E2=80=9D=20
>> or =E2=80=9Ckernel=E2=80=9D as appropriate.  Even =E2=80=9Ckernel=E2=80=9D=
 seems barely necessary.
>=20
> The thing is, the 'noise' from unset bits is actually important=20
> information as well - at least for the major bits: it was a mostly random=20=

> choice that Intel defined '1' for write access and not for read access.=20=

>=20
>=20

That=E2=80=99s why I suggested =E2=80=9Cread,=E2=80=9D in lowercase, for rea=
ds.  Other than that, most of the unset bits are uninteresting. An OOPS is s=
o likely to be a kernel fault that it=E2=80=99s barely worth mentioning, and=
 I even added a whole separate diagnostic for user oopses.  Similarly, I don=
=E2=80=99t think we need to remind the reader that an oops wasn=E2=80=99t an=
 SGX error or that it wasn=E2=80=99t a PK error.  So I think my idea highlig=
hts the interesting bits and avoids distraction from the uninteresting bits.=