Subject: Re: [PATCH RFC v1 2/6] swiotlb: convert variables to arrays

From: Konrad Rzeszutek Wilk <[email-masked]>
Date: Thu, 4 Feb 2021 14:31:36 -0500

On Thu, Feb 04, 2021 at 11:49:23AM +0000, Robin Murphy wrote:
> On 2021-02-04 07:29, Christoph Hellwig wrote:
> > On Wed, Feb 03, 2021 at 03:37:05PM -0800, Dongli Zhang wrote:
> > > This patch converts several swiotlb related variables to arrays, in
> > > order to maintain stat/status for different swiotlb buffers. Here are
> > > variables involved:
> > > 
> > > - io_tlb_start and io_tlb_end
> > > - io_tlb_nslabs and io_tlb_used
> > > - io_tlb_list
> > > - io_tlb_index
> > > - max_segment
> > > - io_tlb_orig_addr
> > > - no_iotlb_memory
> > > 
> > > There is no functional change and this is to prepare to enable 64-bit
> > > swiotlb.
> > 
> > Claire Chang (on Cc) already posted a patch like this a month ago,
> > which looks much better because it actually uses a struct instead
> > of all the random variables.
> 
> Indeed, I skimmed the cover letter and immediately thought that this whole
> thing is just the restricted DMA pool concept[1] again, only from a slightly
> different angle.

Kind of. Let me lay out how some of these pieces are right now:

+-----------------------+      +----------------------+
|                       |      |                      |
|                       |      |                      |
|   a)Xen-SWIOTLB       |      | b)SWIOTLB (for !Xen) |
|                       |      |                      |
+-----------XX----------+      +-------X--------------+
              XXXX             XXXXXXXXX
                 XXXX     XX XXX
                    X   XX
                    XXXX
         +----------XX-----------+
         |                       |
         |                       |
         |   c) SWIOTLB generic  |
         |                       |
         +-----------------------+

Dongli's patches modify the SWIOTLB generic c), and Xen-SWIOTLB a)
parts.

Also see the IOMMU_INIT logic which lays this a bit more deepth
(for example how to enable SWIOTLB on AMD boxes, or IBM with Calgary
IOMMU, etc - see iommu_table.h).

Furtheremore it lays the groundwork to allocate AMD SEV SWIOTLB buffers
later after boot (so that you can stich different pools together).
All the bits are kind of inside of the SWIOTLB code. And also it changes
the Xen-SWIOTLB to do something similar.

The mempool did it similarly by taking the internal parts (aka the
various io_tlb) of SWIOTLB and exposing them out and having
other code:

+-----------------------+      +----------------------+
|                       |      |                      |
|                       |      |                      |
| a)Xen-SWIOTLB         |      | b)SWIOTLB (for !Xen) |
|                       |      |                      |
+-----------XX----------+      +-------X--------------+
              XXXX             XXXXXXXXX
                 XXXX     XX XXX
                    X   XX
                    XXXX
         +----------XX-----------+         +------------------+
         |                       |         | Device tree      |
         |                       +<--------+ enabling SWIOTLB |
         |c) SWIOTLB generic     |         |                  |
         |                       |         | mempool          |
         +-----------------------+         +------------------+

What I was suggesting to Clarie to follow Xen model, that is
do something like this:

+-----------------------+      +----------------------+   +--------------------+
|                       |      |                      |   |                    |
|                       |      |                      |   |                    |
| a)Xen-SWIOTLB         |      | b)SWIOTLB (for !Xen) |   | e) DT-SWIOTLB      |
|                       |      |                      |   |                    |
+-----------XX----------+      +-------X--------------+   +----XX-X------------+
              XXXX             XXXXXXXXX        XXX X X XX X XX
                 XXXX     XX XXX        XXXXXXXX
                    X   XX XXXXXXXXXXXXX
                    XXXXXXXX
         +----------XXX----------+
         |                       |
         |                       |
         |c) SWIOTLB generic     |
         |                       |
         +-----------------------+

so using the SWIOTLB generic parts, and then bolt on top
of the device-tree logic, along with the mempool logic.

But Christopher has an interesting suggestion which is
to squash the all the existing code (a, b, c) all together
and pepper it with various jump-tables.

So:

-----------------------------+
| SWIOTLB:                   |
|                            |
|  a) SWIOTLB (for non-Xen)  |
|  b) Xen-SWIOTLB            |
|  c) DT-SWIOTLB             |
|                            |
|                            |
-----------------------------+

with all the various bits (M2P/P2M for Xen, mempool for ARM,
and normal allocation for BM) in one big file.