Re: [RFC PATCH 3/3] hw/block/nvme: add nvme_inject_state HMP command

[Keith Busch]

On Thu, Feb 11, 2021 at 12:38:48PM +0900, Minwoo Im wrote:
> On 21-02-11 12:00:11, Keith Busch wrote:
> > But I would prefer to see advanced retry tied to real errors that can be
> > retried, like if we got an EBUSY or EAGAIN errno or something like that.
> 
> I have seen a thread [1] about ACRE.  Forgive me If I misunderstood this
> thread or missed something after this thread.  It looks like CRD field in
> the CQE can be set for any NVMe error state which means it *may* depend on
> the device status.

Right! Setting CRD values is at the controller's discretion for any
error status as long as the host enables ACRE.

> And this patch just introduced a internal temporarily error state of
> the controller by returning Command Intrrupted status.

It's just purely synthetic, though. I was hoping something more natural
could trigger the status. That might not provide the deterministic
scenario you're looking for, though.

I'm not completely against using QEMU as a development/test vehicle for
corner cases like this, but we are introducing a whole lot of knobs
recently, and you practically need to be a QEMU developer to even find
them. We probably should step up the documentation in the wiki along
with these types of features.
 
> I think, in this stage, we can go with some errors in the middle of the
> AIO (nvme_aio_err()) for advanced retry.  Shouldn't AIO errors are
> retry-able and supposed to be retried ?

Sure, we can assume that receiving an error in the AIO callback means
the lower layers exhausted available recovery mechanisms.