linux-kernel - Re: arm64 syzbot instances

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Message-ID: <CAFEAcA-s79=4VDSA3TO8tpLUMwJE=HcFT4eZO8L8CCkAAfj8PA@mail.gmail.com>
Date:   Sat, 20 Mar 2021 20:43:25 +0000
From:   Peter Maydell <peter.maydell@...aro.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        syzkaller <syzkaller@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        John Garry <john.garry@...wei.com>,
        Alex Bennée <alex.bennee@...aro.org>
Subject: Re: arm64 syzbot instances

On Fri, 12 Mar 2021 at 09:16, Arnd Bergmann <arnd@...db.de> wrote:
> So it's probably qemu that triggers the 'synchronous external
> abort' when accessing the PCI I/O space, which in turn hints
> towards a bug in qemu. Presumably it only returns data from
> I/O ports that are actually mapped to a device when real hardware
> is supposed to return 0xffffffff when reading from unused I/O ports.

Do you have a reference to the bit of the PCI spec that mandates
this -1/discard behaviour for attempted access to places where
there isn't actually a PCI device mapped ? The spec is pretty
long and hard to read...

(Knowing to what extent this behaviour is mandatory for all
PCI systems/host controllers vs just "it would be nice if the
gpex host controller worked this way" would help in figuring
out where in QEMU to change.)

thanks
-- PMM