[Nouveau] [PATCH RFC v1 0/6] swiotlb: 64-bit DMA buffer

Wed Feb 3 23:37:03 UTC 2021

This RFC is to introduce the 2nd swiotlb buffer for 64-bit DMA access.  The
prototype is based on v5.11-rc6.

The state of the art swiotlb pre-allocates <=32-bit memory in order to meet
the DMA mask requirement for some 32-bit legacy device. Considering most
devices nowadays support 64-bit DMA and IOMMU is available, the swiotlb is
not used for most of times, except:

1. The xen PVM domain requires the DMA addresses to both (1) <= the device
dma mask, and (2) continuous in machine address. Therefore, the 64-bit
device may still require swiotlb on PVM domain.

2. From source code the AMD SME/SEV will enable SWIOTLB_FORCE. As a result
it is always required to allocate from swiotlb buffer even the device dma
mask is 64-bit.

sme_early_init()
-> if (sev_active())
       swiotlb_force = SWIOTLB_FORCE;

Therefore, this RFC introduces the 2nd swiotlb buffer for 64-bit DMA
access.  For instance, the swiotlb_tbl_map_single() allocates from the 2nd
64-bit buffer if the device DMA mask min_not_zero(*hwdev->dma_mask,
hwdev->bus_dma_limit) is 64-bit.  With the RFC, the Xen/AMD will be able to
allocate >4GB swiotlb buffer.

With it being 64-bit, you can (not in this patch set but certainly
possible) allocate this at runtime. Meaning the size could change depending
on the device MMIO buffers, etc.

I have tested the patch set on Xen PVM dom0 boot via QEMU. The dom0 is boot
via:

qemu-system-x86_64 -smp 8 -m 20G -enable-kvm -vnc :9 \
-net nic -net user,hostfwd=tcp::5029-:22 \
-hda disk.img \
-device nvme,drive=nvme0,serial=deudbeaf1,max_ioqpairs=16 \
-drive file=test.qcow2,if=none,id=nvme0 \
-serial stdio

The "swiotlb=65536,1048576,force" is to configure 32-bit swiotlb as 128MB
and 64-bit swiotlb as 2048MB. The swiotlb is enforced.

vm# cat /proc/cmdline 
placeholder root=UUID=4e942d60-c228-4caf-b98e-f41c365d9703 ro text
swiotlb=65536,1048576,force quiet splash

[    5.119877] Booting paravirtualized kernel on Xen
... ...
[    5.190423] software IO TLB: Low Mem mapped [mem 0x0000000234e00000-0x000000023ce00000] (128MB)
[    6.276161] software IO TLB: High Mem mapped [mem 0x0000000166f33000-0x00000001e6f33000] (2048MB)

0x0000000234e00000 is mapped to 0x00000000001c0000 (32-bit machine address)
0x000000023ce00000-1 is mapped to 0x000000000ff3ffff (32-bit machine address)
0x0000000166f33000 is mapped to 0x00000004b7280000 (64-bit machine address)
0x00000001e6f33000-1 is mapped to 0x000000033a07ffff (64-bit machine address)

While running fio for emulated-NVMe, the swiotlb is allocating from 64-bit
io_tlb_used-highmem.

vm# cat /sys/kernel/debug/swiotlb/io_tlb_nslabs
65536
vm# cat /sys/kernel/debug/swiotlb/io_tlb_used
258
vm# cat /sys/kernel/debug/swiotlb/io_tlb_nslabs-highmem
1048576
vm# cat /sys/kernel/debug/swiotlb/io_tlb_used-highmem
58880

I also tested virtio-scsi (with "disable-legacy=on,iommu_platform=true") on
VM with AMD SEV enabled.

qemu-system-x86_64 -enable-kvm -machine q35 -smp 36 -m 20G \
-drive if=pflash,format=raw,unit=0,file=OVMF_CODE.pure-efi.fd,readonly \
-drive if=pflash,format=raw,unit=1,file=OVMF_VARS.fd \
-hda ol7-uefi.qcow2 -serial stdio -vnc :9 \
-net nic -net user,hostfwd=tcp::5029-:22 \
-cpu EPYC -object sev-guest,id=sev0,cbitpos=47,reduced-phys-bits=1 \
-machine memory-encryption=sev0 \
-device virtio-scsi-pci,id=scsi,disable-legacy=on,iommu_platform=true \
-device scsi-hd,drive=disk0 \
-drive file=test.qcow2,if=none,id=disk0,format=qcow2

The "swiotlb=65536,1048576" is to configure 32-bit swiotlb as 128MB and
64-bit swiotlb as 2048MB. We do not need to force swiotlb because AMD SEV
will set SWIOTLB_FORCE.

# cat /proc/cmdline
BOOT_IMAGE=/vmlinuz-5.11.0-rc6swiotlb+ root=/dev/mapper/ol-root ro
crashkernel=auto rd.lvm.lv=ol/root rd.lvm.lv=ol/swap rhgb quiet
LANG=en_US.UTF-8 swiotlb=65536,1048576

[    0.729790] AMD Memory Encryption Features active: SEV
... ...
[    2.113147] software IO TLB: Low Mem mapped [mem 0x0000000073e1e000-0x000000007be1e000] (128MB)
[    2.113151] software IO TLB: High Mem mapped [mem 0x00000004e8400000-0x0000000568400000] (2048MB)

While running fio for virtio-scsi, the swiotlb is allocating from 64-bit
io_tlb_used-highmem.

vm# cat /sys/kernel/debug/swiotlb/io_tlb_nslabs
65536
vm# cat /sys/kernel/debug/swiotlb/io_tlb_used
0
vm# cat /sys/kernel/debug/swiotlb/io_tlb_nslabs-highmem
1048576
vm# cat /sys/kernel/debug/swiotlb/io_tlb_used-highmem
64647

Please let me know if there is any feedback for this idea and RFC.

Dongli Zhang (6):
   swiotlb: define new enumerated type
   swiotlb: convert variables to arrays
   swiotlb: introduce swiotlb_get_type() to calculate swiotlb buffer type
   swiotlb: enable 64-bit swiotlb
   xen-swiotlb: convert variables to arrays
   xen-swiotlb: enable 64-bit xen-swiotlb

 arch/mips/cavium-octeon/dma-octeon.c         |   3 +-
 arch/powerpc/kernel/dma-swiotlb.c            |   2 +-
 arch/powerpc/platforms/pseries/svm.c         |   8 +-
 arch/x86/kernel/pci-swiotlb.c                |   5 +-
 arch/x86/pci/sta2x11-fixup.c                 |   2 +-
 drivers/gpu/drm/i915/gem/i915_gem_internal.c |   4 +-
 drivers/gpu/drm/i915/i915_scatterlist.h      |   2 +-
 drivers/gpu/drm/nouveau/nouveau_ttm.c        |   2 +-
 drivers/mmc/host/sdhci.c                     |   2 +-
 drivers/pci/xen-pcifront.c                   |   2 +-
 drivers/xen/swiotlb-xen.c                    | 123 ++++---
 include/linux/swiotlb.h                      |  49 ++-
 kernel/dma/swiotlb.c                         | 382 +++++++++++++---------
 13 files changed, 363 insertions(+), 223 deletions(-)

Thank you very much!

Dongli Zhang