Re: [RFC 1/2] vfio/pci: keep the prefetchable attribute of a BAR region in VMA

[Lorenzo Pieralisi]

[+Jason, Ben]

On Thu, Apr 29, 2021 at 11:29:05AM -0500, Shanker Donthineni wrote:
> For pass-through device assignment, the ARM64 KVM hypervisor retrieves
> the memory region properties physical address, size, and whether a
> region backed with struct page or not from VMA. The prefetchable
> attribute of a BAR region isn't visible to KVM to make an optimal
> decision for stage2 attributes.
> 
> This patch updates vma->vm_page_prot and maps with write-combine
> attribute if the associated BAR is prefetchable. For ARM64
> pgprot_writecombine() is mapped to memory-type MT_NORMAL_NC which
> has no side effects on reads and multiple writes can be combined.
> 
> Signed-off-by: Shanker Donthineni <sdonthineni@xxxxxxxxxx>
> ---
>  drivers/vfio/pci/vfio_pci.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)

A bit of background information that may be useful:

https://lore.kernel.org/linux-pci/2b539df4c9ec703458e46da2fc879ee3b310b31c.camel@xxxxxxxxxxxxxxxxxxx

Lorenzo

> diff --git a/drivers/vfio/pci/vfio_pci.c b/drivers/vfio/pci/vfio_pci.c
> index 5023e23db3bc..1b734fe1dd51 100644
> --- a/drivers/vfio/pci/vfio_pci.c
> +++ b/drivers/vfio/pci/vfio_pci.c
> @@ -1703,7 +1703,11 @@ 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 (IS_ENABLED(CONFIG_ARM64) &&
> +	    (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;
>  
>  	/*
> -- 
> 2.17.1
>