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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [patch 01/15] x86/irq: Convey vector as argument and not in ptregs
Date:   Wed, 26 Feb 2020 13:35:47 -0800
Message-Id: <0BF722CE-26CF-43AC-A2E4-5C4639794159@amacapital.net>
References: <87k149p0na.fsf@nanos.tec.linutronix.de>
Cc:     Brian Gerst <brgerst@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Juergen Gross <JGross@suse.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>
In-Reply-To: <87k149p0na.fsf@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On Feb 26, 2020, at 12:13 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
> =EF=BB=BFBrian Gerst <brgerst@gmail.com> writes:
>=20
>>> On Tue, Feb 25, 2020 at 6:26 PM Thomas Gleixner <tglx@linutronix.de> wro=
te:
>>>=20
>>> Device interrupts which go through do_IRQ() or the spurious interrupt
>>> handler have their separate entry code on 64 bit for no good reason.
>>>=20
>>> Both 32 and 64 bit transport the vector number through ORIG_[RE]AX in
>>> pt_regs. Further the vector number is forced to fit into an u8 and is
>>> complemented and offset by 0x80 for historical reasons.
>>=20
>> The reason for the 0x80 offset is so that the push instruction only
>> takes two bytes.  This allows each entry stub to be packed into a
>> fixed 8 bytes.  idt_setup_apic_and_irq_gates() assumes this 8-byte
>> fixed length for the stubs, so now every odd vector after 0x80 is
>> broken.
>>=20
>>     508:       6a 7f                   pushq  $0x7f
>>     50a:       e9 f1 08 00 00          jmpq   e00 <common_interrupt>
>>     50f:       90                      nop
>>     510:       68 80 00 00 00          pushq  $0x80
>>     515:       e9 e6 08 00 00          jmpq   e00 <common_interrupt>
>>     51a:       66 0f 1f 44 00 00       nopw   0x0(%rax,%rax,1)
>>     520:       68 81 00 00 00          pushq  $0x81
>>     525:       e9 d6 08 00 00          jmpq   e00 <common_interrupt>
>>     52a:       66 0f 1f 44 00 00       nopw   0x0(%rax,%rax,1)
>>=20
>> The 0x81 vector should start at 0x518, not 0x520.
>=20
> Bah, I somehow missed that big fat comment explaining it. :)
>=20
> Thanks for catching it. So my testing just has been lucky to not hit one
> of those.
>=20
> Now the question is whether we care about the packed stubs or just make
> them larger by using alignment to get rid of this silly +0x80 and
> ~vector fixup later on. The straight forward thing clearly has its charm
> and I doubt it matters in measurable ways.

I agree it probably doesn=E2=80=99t matter. That being said, I have a distin=
ct memory of fixing that asm so it would fail the build if the alignment was=
 off.

>=20
> Thanks,
>=20
>        tglx
>=20
>=20