Re: [PATCH mlx5-next v7 0/4] Dynamically assign MSI-X vectors count

From: Keith Busch <kbusch@kernel.org>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Alexander Duyck <alexander.duyck@gmail.com>,
	Bjorn Helgaas <helgaas@kernel.org>,
	Leon Romanovsky <leon@kernel.org>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Saeed Mahameed <saeedm@nvidia.com>,
	Leon Romanovsky <leonro@nvidia.com>,
	Jakub Kicinski <kuba@kernel.org>,
	linux-pci <linux-pci@vger.kernel.org>,
	linux-rdma@vger.kernel.org, Netdev <netdev@vger.kernel.org>,
	Don Dutile <ddutile@redhat.com>,
	Alex Williamson <alex.williamson@redhat.com>,
	"David S . Miller" <davem@davemloft.net>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: [PATCH mlx5-next v7 0/4] Dynamically assign MSI-X vectors count
Date: Fri, 12 Mar 2021 06:36:37 -0700	[thread overview]
Message-ID: <20210312133637.GA43947@C02WT3WMHTD6> (raw)
In-Reply-To: <20210312130017.GT2356281@nvidia.com>

On Fri, Mar 12, 2021 at 09:00:17AM -0400, Jason Gunthorpe wrote:
> On Thu, Mar 11, 2021 at 06:53:16PM -0800, Alexander Duyck wrote:
> > On Thu, Mar 11, 2021 at 3:21 PM Jason Gunthorpe <jgg@nvidia.com> wrote:
> > >
> > > On Thu, Mar 11, 2021 at 01:49:24PM -0800, Alexander Duyck wrote:
> > > > > We don't need to invent new locks and new complexity for something
> > > > > that is trivially solved already.
> > > >
> > > > I am not wanting a new lock. What I am wanting is a way to mark the VF
> > > > as being stale/offline while we are performing the update. With that
> > > > we would be able to apply similar logic to any changes in the future.
> > >
> > > I think we should hold off doing this until someone comes up with HW
> > > that needs it. The response time here is microseconds, it is not worth
> > > any complexity
> > 
> > I disagree. Take a look at section 8.5.3 in the NVMe document that was
> > linked to earlier:
> > https://nvmexpress.org/wp-content/uploads/NVM-Express-1_4a-2020.03.09-Ratified.pdf
> > 
> > This is exactly what they are doing and I think it makes a ton of
> > sense. Basically the VF has to be taken "offline" before you are
> 
> AFAIK this is internal to the NVMe command protocol, not something we
> can expose generically to the OS. mlx5 has no protocol to "offline" an
> already running VF, for instance.
> 
> The way Leon has it arranged that online/offline scheme has no
> relevance because there is no driver or guest attached to the VF to
> see the online/offline transition.
> 
> I wonder if people actually do offline a NVMe VF from a hypervisor?
> Seems pretty weird.

I agree, that would be weird. I'm pretty sure you can't modify these resources
once you attach the nvme VF to a guest. The resource allocation needs to happen
prior to that.

> > Another way to think of this is that we are essentially pulling a
> > device back after we have already allocated the VFs and we are
> > reconfiguring it before pushing it back out for usage. Having a flag
> > that we could set on the VF device to say it is "under
> > construction"/modification/"not ready for use" would be quite useful I
> > would think.
> 
> Well, yes, the whole SRIOV VF lifecycle is a pretty bad fit for the
> modern world.
> 
> I'd rather not see a half-job on a lifecycle model by hacking in
> random flags. It needs a proper comprehensive design.
> 
> Jason