Re: [Xen-devel] [PATCH] xen/domctl: lower loglevel of XEN_DOMCTL_memory_mapping

[Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>]

On 11/09/15 10:17, Jan Beulich wrote:
>>>> On 11.09.15 at 02:59, <konrad.wilk@xxxxxxxxxx> wrote:
>> If you want a formula I would do:
>>
>> #define MAX_SOCKETS 8
>>
>>  max_pfns = pow(2,(MAX_SOCKETS - (max(nr_iommus(), MAX_SOCKETS)))) * 64;
>>
>> Where nr_iommus would have to be somehow implemented, ditto for pow.
>>
>> This should give you:
>>  8   -> 64
>>  7   -> 128
>>  6   -> 256
>>  5   -> 512
>>  4   -> 1024
>>  3   -> 2048
>>  2   -> 4096
>>  1   -> 16384
> 
> 16k seems excessive as a default. Also - why would this be related
> to the number of sockets? I don't think there's a one-IOMMU-per-
> socket rule; fixed-number-of-IOMMUs-per-node might come closer,
> but there we'd have the problem of what "fixed number" is. Wouldn't
> something as simple as 1024 / nr_iommus() do?
> 
> I also don't follow what cache flushes you talked about earlier: I
> don't think the IOMMUs drive any global cache flushes, and I
> would have thought the size limited IOTLB and (CPU side) cache
> ones should be pretty limited in terms of bus load (unless the TLB
> ones would get converted to global ones due to lacking IOMMU
> capabilities). Is that not the case?

The data cache flushes are caused by the memory_type_changed() call at the
bottom of the XEN_DOMCTL_memory_mapping hypercall, not by the IOMMU code itself.

Malcolm

> 
> Jan
> 


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