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

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Date:   Thu, 28 Nov 2019 00:42:25 +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

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.

Thanks,
Ashish

Tue, Nov 26, 2019 at 07:45:27PM +0100, Christoph Hellwig wrote:
> 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:
> 
> https://nam11.safelinks.protection.outlook.com/?url=http%3A%2F%2Fgit.infradead.org%2Fusers%2Fhch%2Fmisc.git%2Fshortlog%2Frefs%2Fheads%2Fdma_alloc_pages&amp;data=02%7C01%7CAshish.Kalra%40amd.com%7Cc977f3861fdd40b8f06508d772a0cf1b%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637103907325617335&amp;sdata=4FzBxGNqNn36CxpU%2FgQ4socs7InNDgAZlTspBMfUsIw%3D&amp;reserved=0
> 
> 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.