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

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Message-ID: <20200107105458.GA3139@lst.de>
Date:   Tue, 7 Jan 2020 11:54:58 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     David Rientjes <rientjes@...gle.com>,
        Christoph Hellwig <hch@....de>,
        "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 Mon, Jan 06, 2020 at 05:34:00PM +0000, Robin Murphy wrote:
> On 01/01/2020 1:54 am, David Rientjes via iommu wrote:
>> Christoph, Thomas, is something like this (without the diagnosic
>> information included in this patch) acceptable for these allocations?
>> Adding expansion support when the pool is half depleted wouldn't be *that*
>> hard.
>>
>> Or are there alternatives we should consider?  Thanks!
>
> Are there any platforms which require both non-cacheable remapping *and* 
> unencrypted remapping for distinct subsets of devices?
>
> If not (and I'm assuming there aren't, because otherwise this patch is 
> incomplete in covering only 2 of the 3 possible combinations), then 
> couldn't we keep things simpler by just attributing both properties to the 
> single "atomic pool" on the basis that one or the other will always be a 
> no-op? In other words, basically just tweaking the existing "!coherent" 
> tests to "!coherent || force_dma_unencrypted()" and doing 
> set_dma_unencrypted() unconditionally in atomic_pool_init().

I think that would make most sense.