Re: [PATCH v4] vfio-pci: Allow to mmap sub-page MMIO BARs if the mmio page is exclusive

[Yongji Xie <xyjxie@linux.vnet.ibm.com>]

Hi, Alex

On 2016/6/23 6:04, Alex Williamson wrote:

> On Mon, 30 May 2016 21:06:37 +0800
> Yongji Xie <xyjxie@linux.vnet.ibm.com> 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. This will cause some
>> performance issues when we passthrough a PCI device with
>> this kind of BARs. Guest will be not able to handle the mmio
>> accesses to the BARs which leads to mmio emulations in host.
>>
>> However, not all sub-page BARs will share page with other BARs.
>> We should allow to mmap the sub-page MMIO BARs which we can
>> make sure will not share page with other BARs.
>>
>> This patch adds support for this case. And we try to add a
>> dummy resource to reserve the remainder of the page which
>> hot-add device's BAR might be assigned into. But it's not
>> necessary to handle the case when the BAR is not page aligned.
>> Because we can't expect the BAR will be assigned into the same
>> location in a page in guest when we passthrough the BAR. And
>> it's hard to access this BAR in userspace because we have
>> no way to get the BAR's location in a page.
>>
>> Signed-off-by: Yongji Xie <xyjxie@linux.vnet.ibm.com>
>> ---
> Hi Yongji,
>
> On 5/22, message-id
> <201605230345.u4N3dJip043323@mx0a-001b2d01.pphosted.com> you indicated
> you'd post the QEMU code which is enabled by this patch "soon".  Have I
> missed that?  I'm still waiting to see it.  Thanks,
>
> Alex

I posted it on May 24th [1]. Do I need to resend it?

[1] http://lists.nongnu.org/archive/html/qemu-devel/2016-05/msg04125.html

Thanks,
Yongji

>>   drivers/vfio/pci/vfio_pci.c         |   87 ++++++++++++++++++++++++++++++++---
>>   drivers/vfio/pci/vfio_pci_private.h |    8 ++++
>>   2 files changed, 89 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
>> index 188b1ff..3cca2a7 100644
>> --- a/drivers/vfio/pci/vfio_pci.c
>> +++ b/drivers/vfio/pci/vfio_pci.c
>> @@ -110,6 +110,73 @@ static inline bool vfio_pci_is_vga(struct pci_dev *pdev)
>>   	return (pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA;
>>   }
>>   
>> +static void vfio_pci_probe_mmaps(struct vfio_pci_device *vdev)
>> +{
>> +	struct resource *res;
>> +	int bar;
>> +	struct vfio_pci_dummy_resource *dummy_res;
>> +
>> +	INIT_LIST_HEAD(&vdev->dummy_resources_list);
>> +
>> +	for (bar = PCI_STD_RESOURCES; bar <= PCI_STD_RESOURCE_END; bar++) {
>> +		res = vdev->pdev->resource + bar;
>> +
>> +		if (!IS_ENABLED(CONFIG_VFIO_PCI_MMAP))
>> +			goto no_mmap;
>> +
>> +		if (!(res->flags & IORESOURCE_MEM))
>> +			goto no_mmap;
>> +
>> +		/*
>> +		 * The PCI core shouldn't set up a resource with a
>> +		 * type but zero size. But there may be bugs that
>> +		 * cause us to do that.
>> +		 */
>> +		if (!resource_size(res))
>> +			goto no_mmap;
>> +
>> +		if (resource_size(res) >= PAGE_SIZE) {
>> +			vdev->bar_mmap_supported[bar] = true;
>> +			continue;
>> +		}
>> +
>> +		if (!(res->start & ~PAGE_MASK)) {
>> +			/*
>> +			 * Add a dummy resource to reserve the remainder
>> +			 * of the exclusive page in case that hot-add
>> +			 * device's bar is assigned into it.
>> +			 */
>> +			dummy_res = kzalloc(sizeof(*dummy_res), GFP_KERNEL);
>> +			if (dummy_res == NULL)
>> +				goto no_mmap;
>> +
>> +			dummy_res->resource.start = res->end + 1;
>> +			dummy_res->resource.end = res->start + PAGE_SIZE - 1;
>> +			dummy_res->resource.flags = res->flags;
>> +			if (request_resource(res->parent,
>> +						&dummy_res->resource)) {
>> +				kfree(dummy_res);
>> +				goto no_mmap;
>> +			}
>> +			dummy_res->index = bar;
>> +			list_add(&dummy_res->res_next,
>> +					&vdev->dummy_resources_list);
>> +			vdev->bar_mmap_supported[bar] = true;
>> +			continue;
>> +		}
>> +		/*
>> +		 * Here we don't handle the case when the BAR is not page
>> +		 * aligned because we can't expect the BAR will be
>> +		 * assigned into the same location in a page in guest
>> +		 * when we passthrough the BAR. And it's hard to access
>> +		 * this BAR in userspace because we have no way to get
>> +		 * the BAR's location in a page.
>> +		 */
>> +no_mmap:
>> +		vdev->bar_mmap_supported[bar] = false;
>> +	}
>> +}
>> +
>>   static void vfio_pci_try_bus_reset(struct vfio_pci_device *vdev);
>>   static void vfio_pci_disable(struct vfio_pci_device *vdev);
>>   
>> @@ -218,12 +285,15 @@ static int vfio_pci_enable(struct vfio_pci_device *vdev)
>>   		}
>>   	}
>>   
>> +	vfio_pci_probe_mmaps(vdev);
>> +
>>   	return 0;
>>   }
>>   
>>   static void vfio_pci_disable(struct vfio_pci_device *vdev)
>>   {
>>   	struct pci_dev *pdev = vdev->pdev;
>> +	struct vfio_pci_dummy_resource *dummy_res, *tmp;
>>   	int i, bar;
>>   
>>   	/* Stop the device from further DMA */
>> @@ -252,6 +322,13 @@ static void vfio_pci_disable(struct vfio_pci_device *vdev)
>>   		vdev->barmap[bar] = NULL;
>>   	}
>>   
>> +	list_for_each_entry_safe(dummy_res, tmp,
>> +				 &vdev->dummy_resources_list, res_next) {
>> +		list_del(&dummy_res->res_next);
>> +		release_resource(&dummy_res->resource);
>> +		kfree(dummy_res);
>> +	}
>> +
>>   	vdev->needs_reset = true;
>>   
>>   	/*
>> @@ -623,9 +700,7 @@ static long vfio_pci_ioctl(void *device_data,
>>   
>>   			info.flags = VFIO_REGION_INFO_FLAG_READ |
>>   				     VFIO_REGION_INFO_FLAG_WRITE;
>> -			if (IS_ENABLED(CONFIG_VFIO_PCI_MMAP) &&
>> -			    pci_resource_flags(pdev, info.index) &
>> -			    IORESOURCE_MEM && info.size >= PAGE_SIZE) {
>> +			if (vdev->bar_mmap_supported[info.index]) {
>>   				info.flags |= VFIO_REGION_INFO_FLAG_MMAP;
>>   				if (info.index == vdev->msix_bar) {
>>   					ret = msix_sparse_mmap_cap(vdev, &caps);
>> @@ -1049,16 +1124,16 @@ static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
>>   		return -EINVAL;
>>   	if (index >= VFIO_PCI_ROM_REGION_INDEX)
>>   		return -EINVAL;
>> -	if (!(pci_resource_flags(pdev, index) & IORESOURCE_MEM))
>> +	if (!vdev->bar_mmap_supported[index])
>>   		return -EINVAL;
>>   
>> -	phys_len = pci_resource_len(pdev, index);
>> +	phys_len = PAGE_ALIGN(pci_resource_len(pdev, index));
>>   	req_len = vma->vm_end - vma->vm_start;
>>   	pgoff = vma->vm_pgoff &
>>   		((1U << (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT)) - 1);
>>   	req_start = pgoff << PAGE_SHIFT;
>>   
>> -	if (phys_len < PAGE_SIZE || req_start + req_len > phys_len)
>> +	if (req_start + req_len > phys_len)
>>   		return -EINVAL;
>>   
>>   	if (index == vdev->msix_bar) {
>> diff --git a/drivers/vfio/pci/vfio_pci_private.h b/drivers/vfio/pci/vfio_pci_private.h
>> index 016c14a..2128de8 100644
>> --- a/drivers/vfio/pci/vfio_pci_private.h
>> +++ b/drivers/vfio/pci/vfio_pci_private.h
>> @@ -57,9 +57,16 @@ struct vfio_pci_region {
>>   	u32				flags;
>>   };
>>   
>> +struct vfio_pci_dummy_resource {
>> +	struct resource		resource;
>> +	int			index;
>> +	struct list_head	res_next;
>> +};
>> +
>>   struct vfio_pci_device {
>>   	struct pci_dev		*pdev;
>>   	void __iomem		*barmap[PCI_STD_RESOURCE_END + 1];
>> +	bool			bar_mmap_supported[PCI_STD_RESOURCE_END + 1];
>>   	u8			*pci_config_map;
>>   	u8			*vconfig;
>>   	struct perm_bits	*msi_perm;
>> @@ -88,6 +95,7 @@ struct vfio_pci_device {
>>   	int			refcnt;
>>   	struct eventfd_ctx	*err_trigger;
>>   	struct eventfd_ctx	*req_trigger;
>> +	struct list_head	dummy_resources_list;
>>   };
>>   
>>   #define is_intx(vdev) (vdev->irq_type == VFIO_PCI_INTX_IRQ_INDEX)