Re: [Xen-devel] [PATCH v2 02/27] ARM: GICv3: allocate LPI pending and property table

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Thu, 23 Mar 2017, Julien Grall wrote:
> Hi Stefano,
> 
> On 23/03/17 17:45, Stefano Stabellini wrote:
> > On Thu, 23 Mar 2017, Julien Grall wrote:
> > > > So as I mentioned before, I am happy to loose the Kconfig option, but
> > > > then we need some sensible default value. In our case we could be cheeky
> > > > here for now and just use the Linux value, because a Linux Dom0 would be
> > > > the only user. But that doesn't sound very future proof, though this may
> > > > not matter for 4.9.
> > > 
> > > I don't think we need a sensible default value and IHMO there is none. I
> > > would
> > > left the user to decide the exact number.
> > 
> > In that case, the command line parameter becomes mandatory: we need to
> > force the user to specify it as we do for dom0_mem today.
> 
> Not really. We should use the hardware value by default. If a user thinks the
> number allocated is too big for his use case, then it can limit using the
> command line.
>
> Anyway, I will not oppose to make this command option mandatory when ITS is in
> use.

Andre wrote:

  Any redistributor supporting 32 bits worth of LPIs would lead to a
  4GB property table and 512MB pending table allocation.

Let's assume that such scenario is realistic (if it is not, then this
discussion is fruitless), in this case the user most surely is not going
to want to use the hardware provided value. But how can she knows it?
How can she find out that she is wasting too much memory on her system?
Is there an easy and obvious way to know?

She could find out if Xen printed a big warning such as:

  USING 4G OF MEMORY FOR ITS PROPTABLE, CONSIDER PASSING max_lpi_bits to XEN 

but to do that, we need a threshold value in Xen, above which the
hypervisor prints the warning. But if we have a threshold value in Xen,
then we might as well consider making it the default ceiling: Xen uses
the hardware provided value, unless it's greater than threshold, in that
case it uses threshold and prints a warning, for example:

  LIMITING ITS PROPTABLE MEMORY TO 1G, CHANGE IT WITH max_lpi_bits PARAMETER


Does it make sense?

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