Re: [Xen-devel] [PATCH] arm/acpi: hide watchdog timer in GTDT table for dom0

[Julien Grall <julien.grall@xxxxxxx>]

On 30/11/16 14:31, Shanker Donthineni wrote:
> Hi Julien,
Hi Shanker,

> On 11/30/2016 04:29 AM, Julien Grall wrote:
> > Hi Shanker,
> >
> > On 29/11/2016 02:59, Shanker Donthineni wrote:
> > > Either we have to hide the watchdog timer section in GTDT or emulate
> > > watchdog timer block for dom0. Otherwise, system gets panic when
> > > dom0 accesses its MMIO registers. The current XEN doesn't support
> > > virtualization of watchdog timer, so hide the watchdog timer section
> > > for dom0.
> > IHMO, the patch description is not really accurate. You are removing
> > the platform timer array that contains watchdog but also Block Timer.
> >
> > Whilst you mention watchdog, you don't have a word on the Block Timer.
> Sure, I'll include detailed description that has word  'Block Timer'.
>
> > Taking a step back, DOM0 is not able to use it because it does not
> > request to map the memory region (this is the behavior expected for
> > PCI and AMBA devices). So this is a bug in the kernel for me.
> Not all the drivers/modules in the Linux kernel are bounded to a device
> object 'struct device'.  The watchdog timer driver is one of this
> category, neither a PCIe nor a AMBA device.
>
> You can find GTDT watchdog timer code at
> https://lkml.org/lkml/2016/7/25/34.
This link points to an i2c driver. Did you intend to post a different link?

> > Assuming this would be fixed, what would be the drawback to give
> > access to dom0 to the watchdogs?
> Absolutely no problem.
>
> > My worry with that change is what if in the future we decide to expose
> > watchdog to DOM0? Linux will still not be ready, unless we have Xen to
> > map those regions at DOM0 build time. That would break the design we
> > have for ACPI.
> Agree, I was also thinking in the same direction. Let me create a v2
> patch that does stage-2 map of MMIO regions at the time of building the
> dom0 domain to fix the problem.
You didn't understand my point here. The design we agreed for ACPI
support is DOM0 should request the mapping before using any device. This 
is because Xen cannot know in advance the memory attribute to use.
So for me this solution is just a workaround. It does not address the 
real problem that Linux does not request the mapping of the memory 
region before using it. So we will end up with the same problem later if 
a device is neither AMBA, nor PCI.
This is easy to solve because watchdog are described the static tables. 
What if the device is described in ASL?
Regards,

--
Julien Grall

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