Re: [RFC V1 1/5] swiotlb: Support allocating DMA memory from SWIOTLB

On Fri, Jan 12, 2024 at 05:52:47AM +0000, Vishal Annapurve wrote:

Modify SWIOTLB framework to allocate DMA memory always from SWIOTLB.

CVMs use SWIOTLB buffers for bouncing memory when using dma_map_* APIs
to setup memory for IO operations. SWIOTLB buffers are marked as shared
once during early boot.

Buffers allocated using dma_alloc_* APIs are allocated from kernel memory
and then converted to shared during each API invocation. This patch ensures
that such buffers are also allocated from already shared SWIOTLB
regions. This allows enforcing alignment requirements on regions marked
as shared.

But does it work in practice? 

Some devices (like GPUs) require a lot of DMA memory. So with this approach
we would need to have huge SWIOTLB buffer that is unused in most VMs.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov