[RFC PATCH] vfio/pci: map prefetchble bars as writecombine

From: Srinath Mannam <srinath.mannam@broadcom.com>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: ray.jui@broadcom.com, vikram.prakash@broadcom.com,
	scott.branden@broadcom.com, kvm@vger.kernel.org,
	bcm-kernel-feedback-list@broadcom.com,
	linux-kernel@vger.kernel.org,
	Srinath Mannam <srinath.mannam@broadcom.com>
Subject: [RFC PATCH] vfio/pci: map prefetchble bars as writecombine
Date: Fri, 13 Jul 2018 10:26:17 +0530	[thread overview]
Message-ID: <1531457777-11825-1-git-send-email-srinath.mannam@broadcom.com> (raw)

By default all BARs map with VMA access permissions
as pgprot_noncached.

In ARM64 pgprot_noncached is MT_DEVICE_nGnRnE which
is strongly ordered and allows aligned access.
This type of mapping works for NON-PREFETCHABLE bars
containing EP controller registers.
But it restricts PREFETCHABLE bars from doing
unaligned access.

In CMB NVMe drives PREFETCHABLE bars are required to
map as MT_NORMAL_NC to do unaligned access.

Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Vikram Prakash <vikram.prakash@broadcom.com>
---
 drivers/vfio/pci/vfio_pci.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
index b423a30..eff6b65 100644
--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c
@@ -1142,7 +1142,10 @@ 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 (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;
 
 	return remap_pfn_range(vma, vma->vm_start, vma->vm_pgoff,
-- 
2.7.4

