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

[Andre Przywara <andre.przywara@xxxxxxx>]

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?

Cheers,
Andre.


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