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From:   Brian Gerst <brgerst@gmail.com>
Date:   Wed, 26 Feb 2020 00:45:43 -0500
Message-ID: <CAMzpN2ij8ReOXZH00puhzraCGRdKY8qt+TMipd_14_XWTu8xtg@mail.gmail.com>
Subject: Re: [patch 01/15] x86/irq: Convey vector as argument and not in ptregs
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     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>
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On Tue, Feb 25, 2020 at 6:26 PM Thomas Gleixner <tglx@linutronix.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.

--
Brian Gerst