Re: [Xen-devel] [PATCH 1 of 4] Prevent low values of max_pages for domains doing sharing or paging

[Tim Deegan <tim@xxxxxxx>]

At 15:32 +0000 on 16 Feb (1329406348), Jan Beulich wrote:
> >>> On 16.02.12 at 15:45, "Andres Lagar-Cavilla" <andres@xxxxxxxxxxxxxxxx> 
> >>> wrote:
> > But I've seen squeezed set criminally low max_pages value (i.e. 256).
> > Granted, this is squeezed's problem, but shouldn't some sanity checking be
> > wired into the hypervisor? Why should we even allow max_pages < tot_pages?
> 
> The only lower boundary that the hypervisor could (perhaps should)
> enforce is that of not being able to guarantee guest forward progress:
> On x86, up to 2 text pages plus up to 4 data pages, times the number
> of page table levels (i.e. 24 pages on 64-bit).

OT: ISTR the actual number of memory accesses needed to complete one x86
instruction is much crazier than that (e.g. a memory-to-memory copy that
faults on the second page of the destination, pushing most of a stack
frame before hitting the SS limit and double-faulting via a task gate,
&c).  We did try to count it exactly for the shadow pagetables once but
gave up and wildly overestimated instead, (because perf near the limit
would have been too awful anyway).

Tim.

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