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Re: [PATCH v7 04/11] hw/block/nvme: Support allocated CNS command varian

From: Klaus Jensen
Subject: Re: [PATCH v7 04/11] hw/block/nvme: Support allocated CNS command variants
Date: Tue, 20 Oct 2020 10:21:18 +0200

On Oct 19 11:17, Dmitry Fomichev wrote:


> CAP.CSS (together with the I/O Command Set data structure) defines
> what command sets are supported by the controller.
> CC.CSS (together with Set Profile) can be set to enable a subset of
> the available command sets.
> Even if a user configures CC.CSS to e.g. Admin only, NVM namespaces
> will still be attached (and thus marked as active).
> Similarly, if a user configures CC.CSS to e.g. NVM, ZNS namespaces
> will still be attached (and thus marked as active).
> However, any operation from a disabled command set will result in a
> Invalid Command Opcode.

This part of the commit message seems irrelevant to the patch.

> Add a new Boolean namespace property, "attached", to provide the most
> basic namespace attachment support. The default value for this new
> property is true. Also, implement the logic in the new CNS values to
> include/exclude namespaces based on this new property. The only thing
> missing is hooking up the actual Namespace Attachment command opcode,
> which will allow a user to toggle the "attached" flag per namespace.

Without Namespace Attachment support, the sole purpose of this parameter
is to allow unusable namespace IDs to be reported. I have no problems
with adding support for the additional CNS values. They will return
identical responses, but I think that is good enough for now.

When it is not really needed, we should be wary of adding a parameter
that is really hard to get rid of again.

> The reason for not hooking up this command completely is because the
> NVMe specification requires the namespace management command to be
> supported if the namespace attachment command is supported.

There are many ways to support Namespace Management, and there are a lot
of quirks with each of them. Do we use a big blockdev and carve out
namespaces? Then, what are the semantics of an image resize operation?

Do we dynamically create blockdev devices - thats sounds pretty nice,
but might have other quirks and the attachment is not really persistent.

I think at least the "attached" parameter should be x-prefixed, but
better, leave it out for now until we know how we want Namespace
Attachment and Management to be implemented.

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