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

[Julien Grall <julien.grall@xxxxxxx>]

Hi Andre,

On 03/23/2017 06:21 PM, Andre Przywara wrote:
> Hi,
>
> On 23/03/17 18:01, Stefano Stabellini wrote:
> > 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?
> Yes, that is exactly what I was after.
> In contrast to the number of device IDs I think the number of LPI bits
> is _not_ a value that software should use directly, it's more a limit,
> actually a GICv3 implementation choice (how wide the LPI ID fields
> internally are, for instance).
>
> So do we want to limit to 1GB and warn starting at 256MB?
I think Stefano was suggesting to make the command line mandatory.

Cheers,

--
Julien Grall

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