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

From: Shanker Donthineni
Date: Thu Apr 29 2021 - 12:29:53 EST

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(-)