Re: [Qemu-devel] [PATCH v5 00/12] KVM Support for MIPS32 Processors

[Peter Maydell <peter.maydell@linaro.org>]

On 14 July 2014 14:33, James Hogan <james.hogan@imgtec.com> wrote:
> On 10/07/14 13:17, Peter Maydell wrote:
>> More generally, there doesn't really seem to be provision in the
>> KVM KVM_EXIT_MMIO API for returning "this access failed".
>> I guess in theory userspace could do all the "figure out how
>> to adjust CPU state to do exception entry and then run VCPU",
>> but that seems like quite a lot of work which the kernel already
>> knows how to do; is there some way to provide a simpler API
>> that lets userspace just inform the kernel that it needs to
>> fault the access?
>
> Indeed. Paolo's idea would work well I think. A data bus error exception
> would likely be the only sensible error response required other than
> ignoring writes or returning a garbage value for a read (which the
> current KVM MMIO API already allows).

I think we should make the API at least permit returning an
(architecture-specific) error code to the kernel, rather than just
a single boolean "failed" response. For instance on ARM the
fault status registers include a single ExT bit for classifying
the type of an external abort. (The meaning of the bit is
IMPDEF; on the Cortex-A15 it can be used to distinguish
AXI bus DECERR ("decode error", ie the interconnect doesn't
have anything attached at that address) and SLVERR ("slave
error", ie there was a device at that address but it chose to
reject the transaction for some reason, eg unsupported
transfer size, timeout, write to read-only location, FIFO
overrun or just because the device was in a bad mood.)

thanks
-- PMM