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Re: [Xen-devel] [PATCH] [pv-ops domU] support MAXSMP

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] [pv-ops domU] support MAXSMP
From: Andrew Jones <drjones@xxxxxxxxxx>
Date: Fri, 18 Dec 2009 11:24:29 +0100
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On 12/18/2009 11:07 AM, Jan Beulich wrote:
Andrew Jones<drjones@xxxxxxxxxx>  18.12.09 10:31>>>
The MAXSMP config option requires CPUMASK_OFFSTACK, which in turn
requires we init the memory for the maps while we bringing up the cpus.
MAXSMP also increases NR_CPUS to 4096. This increase in size exposed an
issue in the argument construction for mulitcalls from
xen_flush_tlb_others. The args should only need space for the actual
number of cpus, which with xen is currently only up to 32.

I don't think new code should be making assumptions like this anymore,
since Xen already supports higher numbers (it's merely the tools which
so far don't). You're basically trading a compile time detectable large
value on stack for one that can grow large dynamically (and hence
require quite a bit more effort to debug, should it ever overrun the

I say 32 cpus in my description to point out the large difference between NR_CPUS and the actual number used. However, the code shouldn't exceed the limits in multicall until something around 2000 cpus.

Keeping it compile-time is good for the stack analysis, but overly wasteful when using values like 4096, since the expected case is thousands less. If we want to keep it static then we need to change MC_ARGS to also be dependent in some way on NR_CPUS.


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