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

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Date:   Thu, 9 Jan 2020 10:58:14 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Christoph Hellwig <hch@....de>
cc:     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 Thu, 9 Jan 2020, Christoph Hellwig 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.
> 

Thanks Christoph and Robin for the help, we're running some additional 
stress tests to double check that our required amount of memory from this 
pool is accurate.  Once that's done, I'll refresh the patch with th 
suggestions and propose it formally.