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.
