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

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Date:   Tue, 26 Nov 2019 19:45:27 +0100
From:   Christoph Hellwig <hch@....de>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     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>,
        Christoph Hellwig <hch@....de>, Ram Pai <linuxram@...ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        Andy Lutomirski <luto@...nel.org>,
        Brijesh Singh <brijesh.singh@....com>,
        "Kalra, Ashish" <Ashish.Kalra@....com>
Subject: Re: [PATCH 1/1] virtio_ring: fix return code on DMA mapping fails

On Sat, Nov 23, 2019 at 09:39:08AM -0600, Tom Lendacky wrote:
> Ideally, having a pool of shared pages for DMA, outside of standard
> SWIOTLB, might be a good thing.  On x86, SWIOTLB really seems geared
> towards devices that don't support 64-bit DMA. If a device supports 64-bit
> DMA then it can use shared pages that reside anywhere to perform the DMA
> and bounce buffering. I wonder if the SWIOTLB support can be enhanced to
> support something like this, using today's low SWIOTLB buffers if the DMA
> mask necessitates it, otherwise using a dynamically sized pool of shared
> pages that can live anywhere.

I think that can be done relatively easily.  I've actually been thinking
of multiple pool support for a whіle to replace the bounce buffering
in the block layer for ISA devices (24-bit addressing).

I've also been looking into a dma_alloc_pages interface to help people
just allocate pages that are always dma addressable, but don't need
a coherent allocation.  My last version I shared is here:

http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma_alloc_pages

But it turns out this still doesn't work with SEV as we'll always
bounce.  And I've been kinda lost on figuring out a way how to
allocate unencrypted pages that we we can feed into the normal
dma_map_page & co interfaces due to the magic encryption bit in
the address.  I guess we could have a fallback path in the mapping
path and just unconditionally clear that bit in the dma_to_phys
path.