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

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Date:   Thu, 9 Jan 2020 15:31:08 +0100
From:   Christoph Hellwig <hch@....de>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        "Singh, Brijesh" <brijesh.singh@....com>,
        "Grimm, Jon" <jon.grimm@....com>, baekhw@...gle.com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>
Subject: Re: [rfc] dma-mapping: preallocate unencrypted DMA atomic pool

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.