linux-kernel - Re: [patch 01/15] x86/irq: Convey vector as argument and not in ptregs

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Date:   Wed, 26 Feb 2020 13:35:47 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Brian Gerst <brgerst@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <JGross@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [patch 01/15] x86/irq: Convey vector as argument and not in ptregs



> On Feb 26, 2020, at 12:13 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> Brian Gerst <brgerst@...il.com> writes:
> 
>>> On Tue, Feb 25, 2020 at 6:26 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>>> 
>>> Device interrupts which go through do_IRQ() or the spurious interrupt
>>> handler have their separate entry code on 64 bit for no good reason.
>>> 
>>> 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.
>> 
>> 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.
>> 
>>     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)
>> 
>> The 0x81 vector should start at 0x518, not 0x520.
> 
> Bah, I somehow missed that big fat comment explaining it. :)
> 
> Thanks for catching it. So my testing just has been lucky to not hit one
> of those.
> 
> 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’t matter. That being said, I have a distinct memory of fixing that asm so it would fail the build if the alignment was off.

> 
> Thanks,
> 
>        tglx
> 
>