Re: [PATCH 0/5] Share sva domains with all devices bound to a mm

[Tina Zhang]

Hi,

On 8/9/23 17:41, Tian, Kevin wrote:
> > From: Zhang, Tina <tina.zhang@xxxxxxxxx>
> > Sent: Tuesday, August 8, 2023 3:50 PM
> >
> > A sva domain's lifetime begins with binding a device to a mm and ends
> > by releasing all the bound devices from that sva domain. Technically,
> > there could be more than one sva domain identified by the mm PASID for
> > the use of bound devices issuing DMA transactions.
>
> Could you elaborate it with some concrete examples which motivate
> this change?
The motivation is to remove the superfluous IOTLB invalidation in 
current VT-d driver.

Currently, in VT-d driver, due to lacking shared sva domain info, in 
intel_flush_svm_range(), both iotlb and dev-tlb invalidation operations 
are performed per-device. However, difference devices could be behind 
one IOMMU (e.g., four devices are behind one IOMMU) and invoking iotlb 
per-device gives us more iotlb invalidation than necessary (4 iotlb 
invalidation instead of 1). This issue may give more performance impact 
when in a virtual machine guest, as currently we have one virtual VT-d 
for in front of those virtual devices.


This patch fixes this issue by attaching shared sva domain information 
to mm, so that it can be utilized in the mm_notifier_ops callbacks.

Regards,
-Tina

> > To support mm PASID 1:n with sva domains, each mm needs to keep both a
> > reference list of allocated sva domains and the corresponding PASID.
> > However, currently, mm struct only has one pasid field for sva usage,
> > which is used to keep the info of an assigned PASID. That pasid field
> > cannot provide sufficient info to build up the 1:n mapping between PASID
> > and sva domains.
> >
> > This patch-set fills the gap by adding an mm_iommu field[1], whose type is
> > mm_iommu_data struct, to replace the old pasid field. The introduced
> > mm_iommu_data struct keeps info of both a reference list of sva domains
> > and an assigned PASID.
> >
> >
> > [1]: https://lore.kernel.org/linux-iommu/ZIBxPd1%2FJCAle6yP@xxxxxxxxxx/
> >
> >
> > The RFC version of this patch-set is here:
> > https://lore.kernel.org/linux-iommu/20230707013441.365583-1-
> > tina.zhang@xxxxxxxxx/
> >
> > Tina Zhang (5):
> >    iommu: Add mm_get_pasid() helper function
> >    iommu: Call helper function to get assigned pasid value
> >    mm: Add structure to keep sva information
> >    iommu: Support mm PASID 1:n with sva domains
> >    mm: Deprecate pasid field
> >
> >   arch/x86/kernel/traps.c                       |  2 +-
> >   .../iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c   | 12 ++---
> >   drivers/iommu/intel/svm.c                     |  8 +--
> >   drivers/iommu/iommu-sva.c                     | 50 ++++++++++++-------
> >   include/linux/iommu.h                         | 19 +++++--
> >   include/linux/mm_types.h                      |  3 +-
> >   kernel/fork.c                                 |  1 -
> >   mm/init-mm.c                                  |  3 --
> >   8 files changed, 58 insertions(+), 40 deletions(-)
> >
> > --
> > 2.17.1