Re: [PATCH v2 RESEND 4/7] swiotlb: Dynamically allocated bounce buffers

["Petr Tesařík" <petr@tesarici.cz>]

Hi Christoph,

On Tue, 16 May 2023 08:13:09 +0200
Christoph Hellwig <hch@lst.de> wrote:

> On Mon, May 15, 2023 at 07:43:52PM +0000, Michael Kelley (LINUX) wrote:
> > FWIW, I don't think the approach you have implemented here will be
> > practical to use for CoCo VMs (SEV, TDX, whatever else).  The problem
> > is that dma_direct_alloc_pages() and dma_direct_free_pages() must
> > call dma_set_decrypted() and dma_set_encrypted(), respectively.  In CoCo
> > VMs, these calls are expensive because they require a hypercall to the host,
> > and the operation on the host isn't trivial either.  I haven't measured the
> > overhead, but doing a hypercall on every DMA map operation and on
> > every unmap operation has long been something we thought we must
> > avoid.  The fixed swiotlb bounce buffer space solves this problem by
> > doing set_decrypted() in batch at boot time, and never
> > doing set_encrypted().  
> 
> I also suspect it doesn't really scale too well due to the number of
> allocations.  I suspect a better way to implement things would be to
> add more large chunks that are used just like the main swiotlb buffers.
> 
> That is when we run out of space try to allocate another chunk of the
> same size in the background, similar to what we do with the pool in
> dma-pool.c.  This means we'll do a fairly large allocation, so we'll
> need compaction or even CMA to back it up, but the other big upside
> is that it also reduces the number of buffers that need to be checked
> in is_swiotlb_buffer or the free / sync side.

I have considered this approach. The two main issues I ran into were:

1. MAX_ORDER allocations were too small (at least with 4K pages), and
   even then they would often fail.

2. Allocating from CMA did work but only from process context.
   I made a stab at modifying the CMA allocator to work from interrupt
   context, but there are non-trivial interactions with the buddy
   allocator. Making them safe from interrupt context looked like a
   major task.

I also had some fears about the length of the dynamic buffer list. I
observed maximum length for block devices, and then it roughly followed
the queue depth. Walking a few hundred buffers was still fast enough.
I admit the list length may become an issue with high-end NVMe and
I/O-intensive applications.

Last but not least, when many smaller swiotlb chunks are allocated, they
must be kept in a list (or another data structure), somewhat reducing the
performance win. OK, one thing I did *not* consider back then was
allocating these additional swiotlb chunks per device. It looks a bit
too wasteful.

Petr T