Re: [PATCH v3 3/3] tools: introduce parameter max_wp_ram_ranges.

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@suse.com]
> Sent: 03 February 2016 08:33
> To: Zhang Yu
> Cc: Andrew Cooper; Ian Campbell; Paul Durrant; Wei Liu; Ian Jackson; Stefano
> Stabellini; Kevin Tian; zhiyuan.lv@intel.com; xen-devel@lists.xen.org; Keir
> (Xen.org)
> Subject: Re: [Xen-devel] [PATCH v3 3/3] tools: introduce parameter
> max_wp_ram_ranges.
> 
> >>> On 03.02.16 at 08:10, <yu.c.zhang@linux.intel.com> wrote:
> > On 2/2/2016 11:21 PM, Jan Beulich wrote:
> >>>>> On 02.02.16 at 16:00, <yu.c.zhang@linux.intel.com> wrote:
> >>> The limit of 4G is to avoid the data missing from uint64 to uint32
> >>> assignment. And I can accept the 8K limit for XenGT in practice.
> >>> After all, it is vGPU page tables we are trying to trap and emulate,
> >>> not normal page frames.
> >>>
> >>> And I guess the reason that one domain exhausting Xen's memory can
> >>> affect another domain is because rangeset uses Xen heap, instead of
> the
> >>> per-domain memory. So what about we use a 8K limit by now for XenGT,
> >>> and in the future, if a per-domain memory allocation solution for
> >>> rangeset is ready, we do need to limit the rangeset size. Does this
> >>> sounds more acceptable?
> >>
> >> The lower the limit the better (but no matter how low the limit
> >> it won't make this a pretty thing). Anyway I'd still like to wait
> >> for what Ian may further say on this.
> >>
> > Hi Jan, I just had a discussion with my colleague. We believe 8K could
> > be the biggest limit for the write-protected ram ranges. If in the
> > future, number of vGPU page tables exceeds this limit, we will modify
> > our back-end device model to find a trade-off method, instead of
> > extending this limit. If you can accept this value as the upper bound
> > of rangeset, maybe we do not need to add any tool stack parameters, but
> > define a MAX_NR_WR_RAM_RANGES for the write-protected ram
> rangesset. As
> > to other rangesets, we keep their limit as 256. Does this sounds OK? :)
> 
> I'm getting the impression that we're moving in circles. A blanket
> limit above the 256 one for all domains is _not_ going to be
> acceptable; going to 8k will still need host admin consent. With
> your rangeset performance improvement patch, each range is
> going to be tracked by a 40 byte structure (up from 32), which
> already means an overhead increase for all the other ranges. 8k
> of wp ranges implies an overhead beyond 448k (including the
> xmalloc() overhead), which is not _that_ much, but also not
> negligible.
> 

... which means we are still going to need a toolstack parameter to set the limit. We already have a parameter for VRAM size so is having a parameter for max. GTT shadow ranges such a bad thing? Is the fact that the memory comes from xenheap rather than domheap the real problem?

  Paul

> Jan