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

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

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@...adcom.com>
Reviewed-by: Ray Jui <ray.jui@...adcom.com>
Reviewed-by: Vikram Prakash <vikram.prakash@...adcom.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