Re: arm64 syzbot instances
From: Arnd Bergmann <arnd@arndb.de>
Date: 2021-03-21 11:57:37
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On Sat, Mar 20, 2021 at 9:43 PM Peter Maydell [off-list ref] wrote:
On Fri, 12 Mar 2021 at 09:16, Arnd Bergmann [off-list ref] wrote:quoted
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.)
Sorry, I don't. I can probably find something in there myself, but in the end it comes down to Linux drivers relying on this behavior for ISA devices since the start. On an old-style x86 PC, this is the only method for finding out if a device is present or not, since there is no description in the firmware that lists them. PCIe devices remain backwards compatible with the old ISA bus, so the old behavior must generally be kept possible. I don't think a specification for the ISA bus exists at all, and I found nothing in the related LPC specification about reading from an unknown device. https://tldp.org/HOWTO/Plug-and-Play-HOWTO-6.html#ss6.12 states the behavior of the ISA I/O ports and how Linux drivers rely on that. Is that enough for you? Arnd _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel