Re: [RFC PATCH 2/3] vfio-pci: Allow to mmap sub-page MMIO BARs if all MMIO BARs are page aligned

On 2015/12/17 4:04, Alex Williamson wrote:

On Fri, 2015-12-11 at 16:53 +0800, Yongji Xie wrote:

quoted

Current vfio-pci implementation disallows to mmap
sub-page(size < PAGE_SIZE) MMIO BARs because these BARs' mmio page
may be shared with other BARs.

But we should allow to mmap these sub-page MMIO BARs if all MMIO BARs
are page aligned which leads the BARs' mmio page would not be shared
with other BARs.

This patch adds support for this case and we also add a
VFIO_DEVICE_FLAGS_PCI_PAGE_ALIGNED flag to notify userspace that
platform supports all MMIO BARs to be page aligned.

Signed-off-by: Yongji Xie <redacted>
---
  drivers/vfio/pci/vfio_pci.c         |   10 +++++++++-
  drivers/vfio/pci/vfio_pci_private.h |    5 +++++
  include/uapi/linux/vfio.h           |    2 ++
  3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/vfio/pci/vfio_pci.c

b/drivers/vfio/pci/vfio_pci.c
index 32b88bd..dbcad99 100644

--- a/drivers/vfio/pci/vfio_pci.c
+++ b/drivers/vfio/pci/vfio_pci.c

@@ -443,6 +443,9 @@ static long vfio_pci_ioctl(void *device_data,
  		if (vdev->reset_works)
  			info.flags |= VFIO_DEVICE_FLAGS_RESET;
  
+		if (vfio_pci_bar_page_aligned())
+			info.flags |=

VFIO_DEVICE_FLAGS_PCI_PAGE_ALIGNED;
+
  		info.num_regions = VFIO_PCI_NUM_REGIONS;
  		info.num_irqs = VFIO_PCI_NUM_IRQS;
  

@@ -479,7 +482,8 @@ static long vfio_pci_ioctl(void *device_data,
  				     VFIO_REGION_INFO_FLAG_WRITE;
  			if (IS_ENABLED(CONFIG_VFIO_PCI_MMAP) &&
  			    pci_resource_flags(pdev, info.index) &
-			    IORESOURCE_MEM && info.size >=

PAGE_SIZE)
+			    IORESOURCE_MEM && (info.size >=
PAGE_SIZE ||
+			    vfio_pci_bar_page_aligned()))
  				info.flags |=
VFIO_REGION_INFO_FLAG_MMAP;
  			break;
  		case VFIO_PCI_ROM_REGION_INDEX:

@@ -855,6 +859,10 @@ static int vfio_pci_mmap(void *device_data,

struct vm_area_struct *vma)
  		return -EINVAL;
  
  	phys_len = pci_resource_len(pdev, index);
+
+	if (vfio_pci_bar_page_aligned())
+		phys_len = PAGE_ALIGN(phys_len);
+
  	req_len = vma->vm_end - vma->vm_start;
  	pgoff = vma->vm_pgoff &
  		((1U << (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT)) - 1);

diff --git a/drivers/vfio/pci/vfio_pci_private.h

b/drivers/vfio/pci/vfio_pci_private.h
index 0e7394f..319352a 100644

--- a/drivers/vfio/pci/vfio_pci_private.h
+++ b/drivers/vfio/pci/vfio_pci_private.h

@@ -69,6 +69,11 @@ struct vfio_pci_device {
  #define is_irq_none(vdev) (!(is_intx(vdev) || is_msi(vdev) ||

is_msix(vdev)))
  #define irq_is(vdev, type) (vdev->irq_type == type)
  
+static inline bool vfio_pci_bar_page_aligned(void)
+{
+	return IS_ENABLED(CONFIG_PPC64);
+}

I really dislike this.  This is a problem for any architecture that
runs on larger pages, and even an annoyance on 4k hosts.  Why are we
only solving it for PPC64?

Yes, I know it's a problem for other architectures. But I'm not sure if other archs prefer
to enforce the alignment of all BARs to be at least PAGE_SIZE which would result in
some waste of address space.

So I just propose a prototype and add PPC64 support here. And other archs could decide
to use it or not by themselves.

Can't we do something similar in the core PCI code and detect it?

So you mean we can do it like this:

diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index d390fc1..f46c04d 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h

@@ -320,6 +320,11 @@ static inline resource_size_t pci_resource_alignment(struct pci_dev *dev,
        return resource_alignment(res);
 }

+static inline bool pci_bar_page_aligned(void)
+{
+       return IS_ENABLED(CONFIG_PPC64);
+}
+
 void pci_enable_acs(struct pci_dev *dev);

 struct pci_dev_reset_methods {

or add a config option to indicate that PCI MMIO BARs should be page aligned? Thanks.

Regards
Yongji Xie

quoted

+
  extern void vfio_pci_intx_mask(struct vfio_pci_device *vdev);
  extern void vfio_pci_intx_unmask(struct vfio_pci_device *vdev);
  

diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
index 751b69f..1fc8066 100644
--- a/include/uapi/linux/vfio.h
+++ b/include/uapi/linux/vfio.h

@@ -171,6 +171,8 @@ struct vfio_device_info {
  #define VFIO_DEVICE_FLAGS_PCI	(1 << 1)	/* vfio-pci

device */
  #define VFIO_DEVICE_FLAGS_PLATFORM (1 << 2)	/* vfio-platform
device */
  #define VFIO_DEVICE_FLAGS_AMBA  (1 << 3)	/* vfio-amba device
*/
+/* Platform support all PCI MMIO BARs to be page aligned */
+#define VFIO_DEVICE_FLAGS_PCI_PAGE_ALIGNED	(1 << 4)
  	__u32	num_regions;	/* Max region index + 1 */
  	__u32	num_irqs;	/* Max IRQ index + 1 */
  };

Why is this on the device info, shouldn't it be per region?  Do we even
need a flag or can we just set the existing mmap flag with the
clarification that sub-host page size regions can mmap an entire host-
page aligned, sized area in the documentation?  Thanks,

Alex

Yes, you are right! We can just set the existing mmap flag instead of adding a new flag.
I will fix it in the next version. Thanks.

Regards
Yongji Xie