Re: [PATCH v2 3/3] tools: introduce parameter max_wp_ram_ranges.

[David Vrabel <david.vrabel@citrix.com>]

On 22/01/16 03:20, Yu Zhang wrote:
> --- a/docs/man/xl.cfg.pod.5
> +++ b/docs/man/xl.cfg.pod.5
> @@ -962,6 +962,24 @@ FIFO-based event channel ABI support up to 131,071 event channels.
>  Other guests are limited to 4095 (64-bit x86 and ARM) or 1023 (32-bit
>  x86).
>  
> +=item B<max_wp_ram_ranges=N>
> +
> +Limit the maximum write-protected ram ranges that can be tracked
> +inside one ioreq server rangeset.
> +
> +Ioreq server uses a group of rangesets to track the I/O or memory
> +resources to be emulated. Default limit of ranges that one rangeset
> +can allocate is set to a small value, due to the fact that these ranges
> +are allocated in xen heap. Yet for the write-protected ram ranges,
> +there are circumstances under which the upper limit inside one rangeset
> +should exceed the default one. E.g. in XenGT, when tracking the per-
> +process graphic translation tables on intel broadwell platforms, the
> +number of page tables concerned will be several thousand(normally
> +in this case, 8192 could be a big enough value). Not configuring this
> +item, or setting its value to 0 will result in the upper limit set
> +to its default one. Users who set his item explicitly are supposed
> +to know the specific scenarios that necessitate this configuration.

This help text isn't very helpful.  How is a user supposed to "know the
specific scenarios" that need this option?

Why doesn't the toolstack (or qemu) automatically set this value based
on whether GVT-g/GVT-d is being used? Then there is no need to even
present this option to the user.

David