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Re: [qemu-s390x] [Qemu-devel] [PATCH v3 1/6] vfio-ccw: make it safe to a

From: Farhan Ali
Subject: Re: [qemu-s390x] [Qemu-devel] [PATCH v3 1/6] vfio-ccw: make it safe to access channel programs
Date: Tue, 5 Feb 2019 10:14:46 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 02/05/2019 09:48 AM, Eric Farman wrote:

On 02/05/2019 07:35 AM, Cornelia Huck wrote:
On Tue, 5 Feb 2019 12:52:29 +0100
Halil Pasic <address@hidden> wrote:

On Mon, 4 Feb 2019 16:31:02 +0100
Cornelia Huck <address@hidden> wrote:

On Thu, 31 Jan 2019 13:34:55 +0100
Halil Pasic <address@hidden> wrote:
On Thu, 31 Jan 2019 12:52:20 +0100
Cornelia Huck <address@hidden> wrote:
On Wed, 30 Jan 2019 19:51:27 +0100
Halil Pasic <address@hidden> wrote:
On Wed, 30 Jan 2019 14:22:07 +0100
Cornelia Huck <address@hidden> wrote:
When we get a solicited interrupt, the start function may have
been cleared by a csch, but we still have a channel program
structure allocated. Make it safe to call the cp accessors in
any case, so we can call them unconditionally.

I read this like it is supposed to be safe regardless of
parallelism and threads. However I don't see any explicit
synchronization done for cp->initialized.

I've managed to figure out how is that supposed to be safe
for the cp_free() (which is probably our main concern) in
vfio_ccw_sch_io_todo(), but if fail when it comes to the one
in vfio_ccw_mdev_notifier().

Can you explain us how does the synchronization work?

You read that wrong, I don't add synchronization, I just add a check.

Now I'm confused. Does that mean we don't need synchronization for this?

If we lack synchronization (that is not provided by the current state
machine handling, or the rework here), we should do a patch on top
(preferably on top of the whole series, so this does not get even more
tangled up.) This is really just about the extra check.

I'm not a huge fan of keeping or introducing races -- it makes things
difficult to reason about, but I do have some understanging your

The only thing I want to avoid is knowingly making things worse than
before, and I don't think this patch does that.

This patch-series is AFAICT a big improvement over what we have. I would
like Farhan confirming that it makes these hick-ups when he used to hit
BUSY with another ssch request disappear. If it does (I hope it does)
it's definitely a good thing for anybody who wants to use vfio-ccw.

Yep. There remains a lot to be done, but it's a first step.

s/a first step/an excellent first step/  :)

Can't speak for Farhan, but this makes things somewhat better for me. I'm still getting some periodic errors, but they happen infrequently enough now that debugging them is frustrating.  ;-)

  - Eric

I ran the my workloads/tests with the patches and like Eric I notice the errors I previously hit less frequently.

Yet I find it difficult to slap my r-b over racy code, or partial
solutions. In the latter case, when I lack conceptual clarity, I find it
difficult to tell if we are heading into the right direction, or is what
we build today going to turn against us tomorrow. Sorry for being a drag.

As long as we don't introduce bad user space interfaces we have to drag
around forever, I think anything is fair game if we think it's a good
idea at that moment. We can rewrite things if it turned out to be a bad
idea (although I'm not arguing for doing random crap, of course :)

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