Re: [PATCH v2 for-6.0?] hw/pci-host/gpex: Don't fault for unmapped parts of MMIO and PIO windows

[Richard Henderson]

On 3/25/21 10:33 AM, Peter Maydell wrote:
> Currently the gpex PCI controller implements no special behaviour for
> guest accesses to areas of the PIO and MMIO where it has not mapped
> any PCI devices, which means that for Arm you end up with a CPU
> exception due to a data abort.
>
> Most host OSes expect "like an x86 PC" behaviour, where bad accesses
> like this return -1 for reads and ignore writes.  In the interests of
> not being surprising, make host CPU accesses to these windows behave
> as -1/discard where there's no mapped PCI device.
>
> The old behaviour generally didn't cause any problems, because
> almost always the guest OS will map the PCI devices and then only
> access where it has mapped them. One corner case where you will see
> this kind of access is if Linux attempts to probe legacy ISA
> devices via a PIO window access. So far the only case where we've
> seen this has been via the syzkaller fuzzer.
>
> Reported-by: Dmitry Vyukov<dvyukov@google.com>
> Fixes:https://bugs.launchpad.net/qemu/+bug/1918917
> Signed-off-by: Peter Maydell<peter.maydell@linaro.org>
> ---
> v1->v2 changes: put in the hw_compat machinery.
>
> Still not sure if I want to put this in 6.0 or not.

I know what you mean.

>   include/hw/pci-host/gpex.h |  4 +++
>   hw/core/machine.c          |  1 +
>   hw/pci-host/gpex.c         | 56 ++++++++++++++++++++++++++++++++++++--
>   3 files changed, 58 insertions(+), 3 deletions(-)

That said, the code looks fine, so,

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>


r~