linux-kernel - Re: [PATCH] vfio: selftests: Add vfio_dma_mapping_mmio

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Message-ID: <aV7gG5fWY84BtjNy@devgpu015.cco6.facebook.com>
Date: Wed, 7 Jan 2026 14:37:15 -0800
From: Alex Mastro <amastro@...com>
To: Alex Williamson <alex@...zbot.org>, David Matlack <dmatlack@...gle.com>,
        Shuah Khan <shuah@...nel.org>
CC: Peter Xu <peterx@...hat.com>, <linux-kernel@...r.kernel.org>,
        <kvm@...r.kernel.org>, <linux-kselftest@...r.kernel.org>,
        Jason Gunthorpe
	<jgg@...pe.ca>
Subject: Re: [PATCH] vfio: selftests: Add vfio_dma_mapping_mmio_test

On Wed, Jan 07, 2026 at 02:13:28PM -0800, Alex Mastro wrote:
 
> @@ -124,20 +127,43 @@ static void vfio_pci_region_get(struct vfio_pci_device *device, int index,
>  static void vfio_pci_bar_map(struct vfio_pci_device *device, int index)
>  {
>  	struct vfio_pci_bar *bar = &device->bars[index];
> +	size_t align, size;
> +	void *map_base, *map_align;
>  	int prot = 0;
>  
>  	VFIO_ASSERT_LT(index, PCI_STD_NUM_BARS);
>  	VFIO_ASSERT_NULL(bar->vaddr);
>  	VFIO_ASSERT_TRUE(bar->info.flags & VFIO_REGION_INFO_FLAG_MMAP);
> +	VFIO_ASSERT_GT(bar->info.size, 0);
>  
>  	if (bar->info.flags & VFIO_REGION_INFO_FLAG_READ)
>  		prot |= PROT_READ;
>  	if (bar->info.flags & VFIO_REGION_INFO_FLAG_WRITE)
>  		prot |= PROT_WRITE;
>  
> -	bar->vaddr = mmap(NULL, bar->info.size, prot, MAP_FILE | MAP_SHARED,
> +	/*
> +	 * Align the mmap for more efficient IOMMU mapping.
> +	 * The largest PUD size supporting huge pfnmap is 1GiB.
> +	 */
> +	size = bar->info.size;
> +	align = min_t(u64, 1ULL << __builtin_ctzll(size), SZ_1G);
> +
> +	map_base = mmap(NULL, size + align, PROT_NONE,
> +			MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> +	VFIO_ASSERT_NE(map_base, MAP_FAILED);
> +
> +	map_align = (void *)ALIGN((uintptr_t)map_base, align);
> +
> +	if (map_align > map_base)
> +		munmap(map_base, map_align - map_base);
> +	if (align > (size_t)(map_align - map_base))

I realized that this is tautological. Will fix in v2.

> +		munmap(map_align + size, align - (map_align - map_base));
> +
> +	bar->vaddr = mmap(map_align, size, prot, MAP_SHARED | MAP_FIXED,
>  			  device->fd, bar->info.offset);
>  	VFIO_ASSERT_NE(bar->vaddr, MAP_FAILED);
> +
> +	madvise(bar->vaddr, size, MADV_HUGEPAGE);
>  }