[PATCH RFC v1 0/7] swiotlb: extra 64-bit buffer for dev->dma_io_tlb_mem

[Dongli Zhang]

Hello,

I used to send out a patchset on 64-bit buffer and people thought it was
the same as Restricted DMA. However, the 64-bit buffer is still not supported.

https://lore.kernel.org/all/20210203233709.19819-1-dongli.zhang@xxxxxxxxxx/

This RFC is to introduce the extra swiotlb buffer with SWIOTLB_ANY flag,
to support 64-bit swiotlb.

The core ideas are:

1. Create an extra io_tlb_mem with SWIOTLB_ANY flags.

2. The dev->dma_io_tlb_mem is set to either default or the extra io_tlb_mem,
   depending on dma mask.


Would you please help suggest for below questions in the RFC?

- Is it fine to create the extra io_tlb_mem?

- Which one is better: to create a separate variable for the extra
  io_tlb_mem, or make it an array of two io_tlb_mem?

- Should I set dev->dma_io_tlb_mem in each driver (e.g., virtio driver as
  in this patchset)based on the value of
  min_not_zero(*dev->dma_mask, dev->bus_dma_limit), or at higher level
  (e.g., post pci driver)?


This patchset is to demonstrate that the idea works. Since this is just a
RFC, I have only tested virtio-blk on qemu-7.0 by enforcing swiotlb. It is
not tested on AMD SEV environment.

qemu-system-x86_64 -cpu host -name debug-threads=on \
-smp 8 -m 16G -machine q35,accel=kvm -vnc :5 -hda boot.img \
-kernel mainline-linux/arch/x86_64/boot/bzImage \
-append "root=/dev/sda1 init=/sbin/init text console=ttyS0 loglevel=7 swiotlb=327680,3145728,force" \
-device virtio-blk-pci,id=vblk0,num-queues=8,drive=drive0,disable-legacy=on,iommu_platform=true \
-drive file=test.raw,if=none,id=drive0,cache=none \
-net nic -net user,hostfwd=tcp::5025-:22 -serial stdio


The kernel command line "swiotlb=327680,3145728,force" is to allocate 6GB for
the extra swiotlb.

[    2.826676] PCI-DMA: Using software bounce buffering for IO (SWIOTLB)
[    2.826693] software IO TLB: default mapped [mem 0x0000000037000000-0x000000005f000000] (640MB)
[    2.826697] software IO TLB: high mapped [mem 0x00000002edc80000-0x000000046dc80000] (6144MB)

The highmem swiotlb is being used by virtio-blk.

$ cat /sys/kernel/debug/swiotlb/swiotlb-hi/io_tlb_nslabs 
3145728
$ cat /sys/kernel/debug/swiotlb/swiotlb-hi/io_tlb_used 
8960


Dongli Zhang (7):
  swiotlb: introduce the highmem swiotlb buffer
  swiotlb: change the signature of remap function
  swiotlb-xen: support highmem for xen specific code
  swiotlb: to implement io_tlb_high_mem
  swiotlb: add interface to set dev->dma_io_tlb_mem
  virtio: use io_tlb_high_mem if it is active
  swiotlb: fix the slot_addr() overflow

arch/powerpc/kernel/dma-swiotlb.c      |   8 +-
arch/x86/include/asm/xen/swiotlb-xen.h |   2 +-
arch/x86/kernel/pci-dma.c              |   5 +-
drivers/virtio/virtio.c                |   8 ++
drivers/xen/swiotlb-xen.c              |  16 +++-
include/linux/swiotlb.h                |  14 ++-
kernel/dma/swiotlb.c                   | 136 +++++++++++++++++++++-------
7 files changed, 145 insertions(+), 44 deletions(-)

Thank you very much for feedback and suggestion!

Dongli Zhang