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

From: yongji xie <[email-masked]>
Date: Thu, 17 Dec 2015 18:26:16 +0800

On 2015/12/17 4:04, Alex Williamson wrote:
> On Fri, 2015-12-11 at 16:53 +0800, Yongji Xie wrote:
>> 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 <[email-masked]>
>> ---
>>   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
>> +
>>   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