linux-kernel - Re: [PATCH 1/1] virtio_ring: fix return code on DMA mapping fails

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Date:   Fri, 29 Nov 2019 01:57:55 +0000
From:   Ashish Kalra <ashish.kalra@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Halil Pasic <pasic@...ux.ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Cornelia Huck <cohuck@...hat.com>,
        linux-s390@...r.kernel.org, Michael Mueller <mimu@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Ram Pai <linuxram@...ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Andy Lutomirski <luto@...nel.org>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH 1/1] virtio_ring: fix return code on DMA mapping fails

On Thu, Nov 28, 2019 at 08:05:38AM +0100, Christoph Hellwig wrote:
> On Thu, Nov 28, 2019 at 12:42:25AM +0000, Ashish Kalra wrote:
> > Why can't we leverage CMA instead of SWIOTLB for DMA when SEV is
> > enabled, CMA is well integerated with the DMA subsystem and handles
> > encrypted pages when force_dma_unencrypted() returns TRUE. 
> > 
> > Though, CMA might face the same issues as SWIOTLB bounce buffers, it's
> > size is similarly setup statically as SWIOTLB does or can be set as a 
> > percentage of the available system memory.
> 
> How is CMA integrated with SEV?  CMA just gives a contiguous chunk
> of memory, which still needs to be remapped as unencrypted before
> returning it to the user.

Looking at the implementation, i see that CMA is accessible using
dma_alloc_coherent() and the buffer allocated by dma_alloc_coherent()
either using the CMA or the page allocator, will be marked/tagged as
unencrypted and returned to the user.

But for dma_map_xx() interfaces, the memory will need be remapped as
unencrypted memory (as you mentioned).

Thanks,
Ashish