[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v2 02/27] ARM: GICv3: allocate LPI pending and property table
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. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx https://lists.xen.org/xen-devel
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