Re: [PATCH v4 00/14] hw/block/nvme: Support Namespace Types and Zoned Namespace Command Set

[Klaus Jensen <its@irrelevant.dk>]

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On Sep 29 10:29, Keith Busch wrote:
> On Tue, Sep 29, 2020 at 12:46:33PM +0200, Klaus Jensen wrote:
> > It is unmistakably clear that you are invalidating my arguments about
> > portability and endianness issues by suggesting that we just remove
> > persistent state and deal with it later, but persistence is the killer
> > feature that sets the QEMU emulated device apart from other emulation
> > options. It is not about using emulation in production (because yeah,
> > why would you?), but persistence is what makes it possible to develop
> > and test "zoned FTLs" or something that requires recovery at power up.
> > This is what allows testing of how your host software deals with opened
> > zones being transitioned to FULL on power up and the persistent tracking
> > of LBA allocation (in my series) can be used to properly test error
> > recovery if you lost state in the app.
> 
> Hold up -- why does an OPEN zone transition to FULL on power up? The
> spec suggests it should be CLOSED. The spec does appear to support going
> to FULL on a NVM Subsystem Reset, though. Actually, now that I'm looking
> at this part of the spec, these implicit transitions seem a bit less
> clear than I expected. I'm not sure it's clear enough to evaluate qemu's
> compliance right now.
> 
> But I don't see what testing these transitions has to do with having a
> persistent state. You can reboot your VM without tearing down the
> running QEMU instance. You can also unbind the driver or shutdown the
> controller within the running operating system. That should make those
> implicit state transitions reachable in order to exercise your FTL's
> recovery.
> 

Oh dear - don't "spec" with me ;)

NVMe v1.4 Section 7.3.1:

    An NVM Subsystem Reset is initiated when:
      * Main power is applied to the NVM subsystem;
      * A value of 4E564D64h ("NVMe") is written to the NSSR.NSSRC
        field;
      * Requested using a method defined in the NVMe Management
        Interface specification; or
      * A vendor specific event occurs.

In the context of QEMU, "Main power" is tearing down QEMU and starting
it from scratch. Just like on a "real" host, unbinding the driver,
rebooting or shutting down the controller does not cause a subsystem
reset (and does not cause the zones to change state). And since the
device does not indicate support for the optional NSSR.NSSRC register,
that way to initiate a subsystem cannot be used.

The reason for moving to FULL is that write pointer updates are not
persisted on each advancement, only when the zone state changes. So
zones that were opened might have valid data, but invalid write pointer.
So the device transitions them to FULL as it is allowed to.

                                                        QED.

> I agree the persistent state provides conveniences for developers. I
> just don't want to gate ZNS enabling on it either since the core design
> doesn't depend on it.

I just don't see why we cant have the icing on the cake when it is
already there :)

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