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

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Message-ID: <20210429162906.32742-1-sdonthineni@nvidia.com>
Date:   Thu, 29 Apr 2021 11:29:04 -0500
From:   Shanker Donthineni <sdonthineni@...dia.com>
To:     Marc Zyngier <maz@...nel.org>,
        Alex Williamson <alex.williamson@...hat.com>
CC:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        <linux-arm-kernel@...ts.infradead.org>,
        <kvmarm@...ts.cs.columbia.edu>, <linux-kernel@...r.kernel.org>,
        <kvm@...r.kernel.org>, Vikram Sethi <vsethi@...dia.com>,
        Shanker Donthineni <sdonthineni@...dia.com>,
        "Jason Sequeira" <jsequeira@...dia.com>
Subject: [RFC 0/2] [RFC] Honor PCI prefetchable attributes for a virtual machine on ARM64

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(-)

-- 
2.17.1