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

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, May 16, 2023 at 08:39:42AM +0200, Petr Tesařík wrote:
> 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.

Can you kick off a worker thread when the swiotlb allocation gets past
some reserve limit? It still has a risk of failing to bounce until the
swiotlb buffer is extended.

> 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.

You could replace the list with an rbtree, O(log n) look-up vs O(n),
could be faster if you have many bounces active.

-- 
Catalin