Re: [rfc] dma-mapping: preallocate unencrypted DMA atomic pool

[Christoph Hellwig]

On Tue, Jan 07, 2020 at 11:57:24AM -0800, David Rientjes wrote:
> I'll rely on Thomas to chime in if this doesn't make sense for the SEV 
> usecase.
> 
> I think the sizing of the single atomic pool needs to be determined.  Our 
> peak usage that we have measured from NVMe is ~1.4MB and atomic_pool is 
> currently sized to 256KB by default.  I'm unsure at this time if we need 
> to be able to dynamically expand this pool with a kworker.
>
> Maybe when CONFIG_AMD_MEM_ENCRYPT is enabled this atomic pool should be 
> sized to 2MB or so and then when it reaches half capacity we schedule some 
> background work to dynamically increase it?  That wouldn't be hard unless 
> the pool can be rapidly depleted.
> 

Note that a non-coherent architecture with the same workload would need
the same size.

> Do we want to increase the atomic pool size by default and then do 
> background dynamic expansion?

For now I'd just scale with system memory size.