linux-kernel - [RFC 1/2] vfio/pci: keep the prefetchable attribute of a BAR region in VMA

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Date:   Thu, 29 Apr 2021 11:29:05 -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 1/2] vfio/pci: keep the prefetchable attribute of a BAR region in VMA

For pass-through device assignment, the ARM64 KVM hypervisor retrieves
the memory region properties physical address, size, and whether a
region backed with struct page or not from VMA. The prefetchable
attribute of a BAR region isn't visible to KVM to make an optimal
decision for stage2 attributes.

This patch updates vma->vm_page_prot and maps with write-combine
attribute if the associated BAR is prefetchable. For ARM64
pgprot_writecombine() is mapped to memory-type MT_NORMAL_NC which
has no side effects on reads and multiple writes can be combined.

Signed-off-by: Shanker Donthineni <sdonthineni@...dia.com>
---
 drivers/vfio/pci/vfio_pci.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index 5023e23db3bc..1b734fe1dd51 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -1703,7 +1703,11 @@ static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
 	}
 
 	vma->vm_private_data = vdev;
-	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
+	if (IS_ENABLED(CONFIG_ARM64) &&
+	    (pci_resource_flags(pdev, index) & IORESOURCE_PREFETCH))
+		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
+	else
+		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
 	vma->vm_pgoff = (pci_resource_start(pdev, index) >> PAGE_SHIFT) + pgoff;
 
 	/*
-- 
2.17.1