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

[Ashish Kalra <ashish.kalra@amd.com>]

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&data=02%7C01%7CAshish.Kalra%40amd.com%7Cc977f3861fdd40b8f06508d772a0cf1b%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637103907325617335&sdata=4FzBxGNqNn36CxpU%2FgQ4socs7InNDgAZlTspBMfUsIw%3D&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.