linux-kernel - Re: [PATCH] PCI: hv: Only reuse existing IRTE allocation for Multi-MSI

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Date:   Wed, 17 Aug 2022 09:51:24 +0200
From:   Lorenzo Pieralisi <lpieralisi@...nel.org>
To:     Dexuan Cui <decui@...rosoft.com>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        "quic_jhugo@...cinc.com" <quic_jhugo@...cinc.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        "robh@...nel.org" <robh@...nel.org>, "kw@...ux.com" <kw@...ux.com>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        Boqun Feng <Boqun.Feng@...rosoft.com>,
        Carl Vanderlip <quic_carlv@...cinc.com>
Subject: Re: [PATCH] PCI: hv: Only reuse existing IRTE allocation for
 Multi-MSI

On Tue, Aug 16, 2022 at 09:13:26PM +0000, Dexuan Cui wrote:
> > From: Bjorn Helgaas <helgaas@...nel.org>
> > Sent: Tuesday, August 16, 2022 8:51 AM
> > To: Dexuan Cui <decui@...rosoft.com>
> > 
> > This has only observations with no explanations, and I don't see how
> > it will be useful to future readers of the git history.
> Please see the below.
>  
> > I assume you bisected the problem to b4b77778ecc5?  
> Yes.
> 
> > Can you just revert that?  A revert requires no more explanation than
> >  "this broke something."
> 
> It's better to not revert b4b77778ecc5, which is required by Jeff's
> Multi-MSI device, which doesn't seem to be affected by the interrupt
> issue I described.

You must debug it, there are no two ways about it.

We can't apply fixes on a hunch, more so given that I am not convinced
at all this patch is fixing anything, it is just papering over an
underlying bug that is still to be pinpointed.

> > I guess this is a fine distinction, but I really don't like random
> > code changes that "seem to avoid a problem but we don't know how."
> > A revert at least has the advantage that we can cover our eyes and
> > pretend the commit never happened. This patch feels like future
> > readers will have to try to understand the code even though we
> > clearly don't understand why it makes a difference.
> 
> I just replied to Lorenzo's email with more details. FYI, this is the link
> to my reply:
> https://lwn.net/ml/linux-kernel/SA1PR21MB1335D08F987BBAE08EADF010BF6B9@SA1PR21MB1335.namprd21.prod.outlook.com/
>  
> I just felt the commit message might be too long if I had put all the
> details there. :-) Can we add a Links: tag?

Commit logs must describe the issue you are fixing, thouroughly and
concisely. To start with "Jeffrey's 4 recent patches" is a very bad
start for a commit log, it means nothing, try to read your log as
someone who needs to understand the commit years down the line please.

Now, back to this patch: we are at -rc1, unless Bjorn is willing to
do so I am not inclined to apply this patch till next merge window
(and actually I am not inclined to merge it at all).

This gives you folks time to debug it (and it must be debugged), the
fact that it works for one multi-MSI device does not mean that the
bug isn't still there - I am worried that the issue is with
b4b77778ecc5 and the interaction with core MSI/IOMMU.

Thanks,
Lorenzo