Message-ID: <20151015194219.GG21930@cbox>
Date:	Thu, 15 Oct 2015 21:42:19 +0200
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Eric Auger <eric.auger@...aro.org>, Arnd Bergmann <arnd@...db.de>,
	linux-arm-kernel@...ts.infradead.org, eric.auger@...com,
	b.reynal@...tualopensystems.com, kvmarm@...ts.cs.columbia.edu,
	kvm@...r.kernel.org, thomas.lendacky@....com, patches@...aro.org,
	suravee.suthikulpanit@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] VFIO: platform: AMD xgbe reset module

On Thu, Oct 15, 2015 at 10:53:17AM -0600, Alex Williamson wrote:
> On Thu, 2015-10-15 at 16:46 +0200, Eric Auger wrote:
> > Hi Arnd,
> > On 10/15/2015 03:59 PM, Arnd Bergmann wrote:
> > > On Thursday 15 October 2015 14:12:28 Christoffer Dall wrote:
> > >>>
> > >>> enum vfio_platform_op {
> > >>>       VFIO_PLATFORM_BIND,
> > >>>       VFIO_PLATFORM_UNBIND,
> > >>>       VFIO_PLATFORM_RESET,
> > >>> };
> > >>>
> > >>> struct platform_driver {
> > >>>         int (*probe)(struct platform_device *);
> > >>>         int (*remove)(struct platform_device *);
> > >>>       ...
> > >>>       int (*vfio_manage)(struct platform_device *, enum vfio_platform_op);
> > >>>         struct device_driver driver;
> > >>> };
> > >>>
> > >>> This would integrate much more closely into the platform driver framework,
> > >>> just like the regular vfio driver integrates into the PCI framework.
> > >>> Unlike PCI however, you can't just use the generic driver framework to
> > >>> unbind the driver, because you still need device specific code.
> > >>>
> > >> Thanks for these suggestions, really helpful.
> > >>
> > >> What I don't understand in the latter example is how VFIO knows which
> > >> struct platform_driver to interact with?
> > > 
> > > This would assume that the driver remains bound to the device, so VFIO
> > > gets a pointer to the device from somewhere (as it does today) and then
> > > follows the dev->driver pointer to get to the platform_driver.
> 
> The complexity of managing a bi-modal driver seems like far more than a
> little bit of code duplication in a device specific reset module and
> extends into how userspace makes devices available through vfio, so I
> think it's too late for that discussion.
>   

I have had extremely limited exposure to the implementation details of
the drivers for devices relevant for VFIO platform, so apologies for
asking stupid questions.

I'm sure that your point is valid, I just fully understand how the
complexities of a bi-modal driver arise?

Is it simply that the reset function in a particular device driver may
not be self-contained so therefore the whole driver would need to be
refactored to be able to do a reset for the purpose of VFIO?

> > >> Also, just so I'm sure I understand correctly, VFIO_PLATFORM_UNBIND is
> > >> then called by VFIO before the VFIO driver unbinds from the device
> > >> (unbinding the platform driver from the device being a completely
> > >> separate thing)?
> > > 
> > > This is where we'd need a little more changes for this approach. Instead
> > > of unbinding the device from its driver, the idea would be that the
> > > driver remains bound as far as the driver model is concerned, but
> > > it would be in a quiescent state where no other subsystem interacts with
> > > it (i.e. it gets unregistered from networking core or whichever it uses).
> > 
> > Currently we use the same mechanism as for PCI, ie. unbind the native
> > driver and then bind VFIO platform driver in its place. Don't you think
> > changing this may be a pain for user-space tools that are designed to
> > work that way for PCI?
> > 
> > My personal preference would be to start with your first proposal since
> > it looks (to me) less complex and "unknown" that the 2d approach.
> > 
> > Let's wait for Alex opinion too...
> 
> I thought the reason we took the approach we have now is so that we
> don't have reset code loaded into the kernel unless we have a device
> that needs it.  Therefore we don't really want to preemptively load all
> the reset drivers and have them do a registration.  The unfortunate
> side-effect of that is the platform code needs to go looking for the
> driver. 

Does the current approach have a separate driver for doing VFIO reset or
does it reuse the existing device driver?

Why does the driver registering itself instead of using symbol_get
imply that we must load reset drivers that we don't need?

Thanks,
-Christoffer


> We do that via the __symbol_get() trick, which only fails
> without modules because the underscore variant isn't defined in that
> case.  I remember asking Eric previously why we're using that rather
> than symbol_get(), I've since forgotten his answer, but the fact that
> __symbol_get() is only defined for modules makes it moot, we either need
> to make symbol_get() work or define __symbol_get() for non-module
> builds.
> 
> Otherwise, we should probably abandon the idea of these reset functions
> being modules and build them into the vfio platform driver (which would
> still be less loaded, dead code than a bi-modal host driver).  Thanks,
> 
> Alex
> 
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