[RFC 0/2] [RFC] Honor PCI prefetchable attributes for a virtual machine on ARM64

[Shanker Donthineni]

Problem statement: Virtual machine crashes when NVIDIA GPU driver access a prefetchable BAR space due to the unaligned reads/writes for pass-through devices. The same binary works fine as expected in the host kernel. Only one BAR has control & status registers (CSR) and other PCI BARs are marked as prefetchable. NVIDIA GPU driver uses the write-combine feature for mapping the prefetchable BARs to improve performance. This problem applies to all other drivers which want to enable WC.
 
Solution: Honor PCI prefetchable attributes for the guest operating systems.
 
Proposal: ARM64-KVM uses VMA struct for the needed information e.g. region physical address, size, and memory-type (struct page backed mapping or anonymous memory) for setting up a stage-2 page table. Right now memory region either can be mapped as DEVICE (strongly ordered) or NORMAL (write-back cache) depends on the flag VM_PFNMAP in VMA. VFIO-PCI will keep the prefetchable (write-combine) information in vma->vm_page_prot similar to other fields, and KVM will prepare stage-2 entries based on the memory-type attribute that was set in VMA.

Shanker Donthineni (2):
  vfio/pci: keep the prefetchable attribute of a BAR region in VMA
  KVM: arm64: Add write-combine support for stage-2 entries

 arch/arm64/include/asm/kvm_mmu.h     |  3 ++-
 arch/arm64/include/asm/kvm_pgtable.h |  2 ++
 arch/arm64/include/asm/memory.h      |  4 +++-
 arch/arm64/kvm/hyp/pgtable.c         |  9 +++++++--
 arch/arm64/kvm/mmu.c                 | 22 +++++++++++++++++++---
 arch/arm64/kvm/vgic/vgic-v2.c        |  2 +-
 drivers/vfio/pci/vfio_pci.c          |  6 +++++-
 7 files changed, 39 insertions(+), 9 deletions(-)

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2.17.1