linux-kernel - Re: [PATCH 1/2] mm: move force_dma_unencrypted() to mem

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Date:   Thu, 20 Feb 2020 18:00:02 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Halil Pasic <pasic@...ux.ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        linux-s390@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Janosch Frank <frankja@...ux.ibm.com>,
        Viktor Mihajlovski <mihajlov@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Ram Pai <linuxram@...ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        David Gibson <david@...son.dropbear.id.au>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Michael Mueller <mimu@...ux.ibm.com>
Subject: Re: [PATCH 1/2] mm: move force_dma_unencrypted() to mem_encrypt.h



On 20.02.20 17:31, Christoph Hellwig wrote:
> On Thu, Feb 20, 2020 at 05:23:20PM +0100, Christian Borntraeger wrote:
>> >From a users perspective it makes absolutely perfect sense to use the
>> bounce buffers when they are NEEDED. 
>> Forcing the user to specify iommu_platform just because you need bounce buffers
>> really feels wrong. And obviously we have a severe performance issue
>> because of the indirections.
> 
> The point is that the user should not have to specify iommu_platform.

I (and Halil) agree on that. From a user perspective we want to
have the right thing in the guest without any command option. The iommu_plaform
thing was indeed a first step to make things work. 

I might mis-read Halils patch, but isnt one effect of this patch set 
that things WILL work without iommu_platform?


> We need to make sure any new hypervisor (especially one that might require
> bounce buffering) always sets it, as was a rather bogus legacy hack
> that isn't extensibe for cases that for example require bounce buffering.