[PATCH v2 0/2] vfio/pci: Fix issues with qword access

[PATCH v2 0/2] vfio/pci: Fix issues with qword access

[Kevin Tian]

Certain devices (e.g. Intel X710) don't support qword access to the
rom bar, otherwise PCI aer errors are observed. Fix it by disabling
the qword access to the rom bar.

While at it, also restrict accesses to the legacy VGA resource to
dword. More for paranoia so stable kernel is not CC-ed.

v2:
- Rebase to 6.19-rc1
- Use enum to avoid adding another bool arg (Alex)
- Elaborate the commit msg (Alex)
- New patch to disallow qword access to legacy VGA (Alex)

v1:
https://lore.kernel.org/all/20251212020941.338355-1-kevin.tian@intel.com/

Kevin Tian (2):
  vfio/pci: Disable qword access to the PCI ROM bar
  vfio/pci: Disable qword access to the VGA region

 drivers/vfio/pci/nvgrace-gpu/main.c |  4 ++--
 drivers/vfio/pci/vfio_pci_rdwr.c    | 25 ++++++++++++++++++-------
 include/linux/vfio_pci_core.h       | 10 +++++++++-
 3 files changed, 29 insertions(+), 10 deletions(-)

-- 
2.43.0

[PATCH v2 1/2] vfio/pci: Disable qword access to the PCI ROM bar

[Kevin Tian]

Commit 2b938e3db335 ("vfio/pci: Enable iowrite64 and ioread64 for vfio
pci") enables qword access to the PCI bar resources. However certain
devices (e.g. Intel X710) are observed with problem upon qword accesses
to the rom bar, e.g. triggering PCI aer errors.

This is triggered by Qemu which caches the rom content by simply does a
pread() of the remaining size until it gets the full contents. The other
bars would only perform operations at the same access width as their
guest drivers.

Instead of trying to identify all broken devices, universally disable
qword access to the rom bar i.e. going back to the old way which worked
reliably for years.

Reported-by: Farrah Chen <farrah.chen@intel.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220740
Fixes: 2b938e3db335 ("vfio/pci: Enable iowrite64 and ioread64 for vfio pci")
Cc: stable@vger.kernel.org
Signed-off-by: Kevin Tian <kevin.tian@intel.com>
---
 drivers/vfio/pci/nvgrace-gpu/main.c |  4 ++--
 drivers/vfio/pci/vfio_pci_rdwr.c    | 25 ++++++++++++++++++-------
 include/linux/vfio_pci_core.h       | 10 +++++++++-
 3 files changed, 29 insertions(+), 10 deletions(-)

diff --git a/drivers/vfio/pci/nvgrace-gpu/main.c b/drivers/vfio/pci/nvgrace-gpu/main.c
index 84d142a47ec6..b45a24d00387 100644
--- a/drivers/vfio/pci/nvgrace-gpu/main.c
+++ b/drivers/vfio/pci/nvgrace-gpu/main.c
@@ -561,7 +561,7 @@ nvgrace_gpu_map_and_read(struct nvgrace_gpu_pci_core_device *nvdev,
 		ret = vfio_pci_core_do_io_rw(&nvdev->core_device, false,
 					     nvdev->resmem.ioaddr,
 					     buf, offset, mem_count,
-					     0, 0, false);
+					     0, 0, false, VFIO_PCI_IO_WIDTH_8);
 	}
 
 	return ret;
@@ -693,7 +693,7 @@ nvgrace_gpu_map_and_write(struct nvgrace_gpu_pci_core_device *nvdev,
 		ret = vfio_pci_core_do_io_rw(&nvdev->core_device, false,
 					     nvdev->resmem.ioaddr,
 					     (char __user *)buf, pos, mem_count,
-					     0, 0, true);
+					     0, 0, true, VFIO_PCI_IO_WIDTH_8);
 	}
 
 	return ret;
diff --git a/drivers/vfio/pci/vfio_pci_rdwr.c b/drivers/vfio/pci/vfio_pci_rdwr.c
index 6192788c8ba3..25380b7dfe18 100644
--- a/drivers/vfio/pci/vfio_pci_rdwr.c
+++ b/drivers/vfio/pci/vfio_pci_rdwr.c
@@ -135,7 +135,8 @@ VFIO_IORDWR(64)
 ssize_t vfio_pci_core_do_io_rw(struct vfio_pci_core_device *vdev, bool test_mem,
 			       void __iomem *io, char __user *buf,
 			       loff_t off, size_t count, size_t x_start,
-			       size_t x_end, bool iswrite)
+			       size_t x_end, bool iswrite,
+			       enum vfio_pci_io_width max_width)
 {
 	ssize_t done = 0;
 	int ret;
@@ -150,20 +151,19 @@ ssize_t vfio_pci_core_do_io_rw(struct vfio_pci_core_device *vdev, bool test_mem,
 		else
 			fillable = 0;
 
-		if (fillable >= 8 && !(off % 8)) {
+		if (fillable >= 8 && !(off % 8) && max_width >= 8) {
 			ret = vfio_pci_iordwr64(vdev, iswrite, test_mem,
 						io, buf, off, &filled);
 			if (ret)
 				return ret;
 
-		} else
-		if (fillable >= 4 && !(off % 4)) {
+		} else if (fillable >= 4 && !(off % 4) && max_width >= 4) {
 			ret = vfio_pci_iordwr32(vdev, iswrite, test_mem,
 						io, buf, off, &filled);
 			if (ret)
 				return ret;
 
-		} else if (fillable >= 2 && !(off % 2)) {
+		} else if (fillable >= 2 && !(off % 2) && max_width >= 2) {
 			ret = vfio_pci_iordwr16(vdev, iswrite, test_mem,
 						io, buf, off, &filled);
 			if (ret)
@@ -234,6 +234,7 @@ ssize_t vfio_pci_bar_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 	void __iomem *io;
 	struct resource *res = &vdev->pdev->resource[bar];
 	ssize_t done;
+	enum vfio_pci_io_width max_width = VFIO_PCI_IO_WIDTH_8;
 
 	if (pci_resource_start(pdev, bar))
 		end = pci_resource_len(pdev, bar);
@@ -262,6 +263,16 @@ ssize_t vfio_pci_bar_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 		if (!io)
 			return -ENOMEM;
 		x_end = end;
+
+		/*
+		 * Certain devices (e.g. Intel X710) don't support qword
+		 * access to the ROM bar. Otherwise PCI AER errors might be
+		 * triggered.
+		 *
+		 * Disable qword access to the ROM bar universally, which
+		 * worked reliably for years before qword access is enabled.
+		 */
+		max_width = VFIO_PCI_IO_WIDTH_4;
 	} else {
 		int ret = vfio_pci_core_setup_barmap(vdev, bar);
 		if (ret) {
@@ -278,7 +289,7 @@ ssize_t vfio_pci_bar_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 	}
 
 	done = vfio_pci_core_do_io_rw(vdev, res->flags & IORESOURCE_MEM, io, buf, pos,
-				      count, x_start, x_end, iswrite);
+				      count, x_start, x_end, iswrite, max_width);
 
 	if (done >= 0)
 		*ppos += done;
@@ -352,7 +363,7 @@ ssize_t vfio_pci_vga_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 	 * to the memory enable bit in the command register.
 	 */
 	done = vfio_pci_core_do_io_rw(vdev, false, iomem, buf, off, count,
-				      0, 0, iswrite);
+				      0, 0, iswrite, VFIO_PCI_IO_WIDTH_8);
 
 	vga_put(vdev->pdev, rsrc);
 
diff --git a/include/linux/vfio_pci_core.h b/include/linux/vfio_pci_core.h
index 706877f998ff..1ac86896875c 100644
--- a/include/linux/vfio_pci_core.h
+++ b/include/linux/vfio_pci_core.h
@@ -145,6 +145,13 @@ struct vfio_pci_core_device {
 	struct list_head	dmabufs;
 };
 
+enum vfio_pci_io_width {
+	VFIO_PCI_IO_WIDTH_1 = 1,
+	VFIO_PCI_IO_WIDTH_2 = 2,
+	VFIO_PCI_IO_WIDTH_4 = 4,
+	VFIO_PCI_IO_WIDTH_8 = 8,
+};
+
 /* Will be exported for vfio pci drivers usage */
 int vfio_pci_core_register_dev_region(struct vfio_pci_core_device *vdev,
 				      unsigned int type, unsigned int subtype,
@@ -188,7 +195,8 @@ pci_ers_result_t vfio_pci_core_aer_err_detected(struct pci_dev *pdev,
 ssize_t vfio_pci_core_do_io_rw(struct vfio_pci_core_device *vdev, bool test_mem,
 			       void __iomem *io, char __user *buf,
 			       loff_t off, size_t count, size_t x_start,
-			       size_t x_end, bool iswrite);
+			       size_t x_end, bool iswrite,
+			       enum vfio_pci_io_width max_width);
 bool __vfio_pci_memory_enabled(struct vfio_pci_core_device *vdev);
 bool vfio_pci_core_range_intersect_range(loff_t buf_start, size_t buf_cnt,
 					 loff_t reg_start, size_t reg_cnt,
-- 
2.43.0

Re: [PATCH v2 1/2] vfio/pci: Disable qword access to the PCI ROM bar

[Chen, Farrah]

On 12/18/2025 4:16 PM, Kevin Tian wrote:
> Commit 2b938e3db335 ("vfio/pci: Enable iowrite64 and ioread64 for vfio
> pci") enables qword access to the PCI bar resources. However certain
> devices (e.g. Intel X710) are observed with problem upon qword accesses
> to the rom bar, e.g. triggering PCI aer errors.
> 
> This is triggered by Qemu which caches the rom content by simply does a
> pread() of the remaining size until it gets the full contents. The other
> bars would only perform operations at the same access width as their
> guest drivers.
> 
> Instead of trying to identify all broken devices, universally disable
> qword access to the rom bar i.e. going back to the old way which worked
> reliably for years.
> 
> Reported-by: Farrah Chen <farrah.chen@intel.com>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=220740
> Fixes: 2b938e3db335 ("vfio/pci: Enable iowrite64 and ioread64 for vfio pci")
> Cc: stable@vger.kernel.org
> Signed-off-by: Kevin Tian <kevin.tian@intel.com>
> ---
>   drivers/vfio/pci/nvgrace-gpu/main.c |  4 ++--
>   drivers/vfio/pci/vfio_pci_rdwr.c    | 25 ++++++++++++++++++-------
>   include/linux/vfio_pci_core.h       | 10 +++++++++-
>   3 files changed, 29 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/vfio/pci/nvgrace-gpu/main.c b/drivers/vfio/pci/nvgrace-gpu/main.c
> index 84d142a47ec6..b45a24d00387 100644
> --- a/drivers/vfio/pci/nvgrace-gpu/main.c
> +++ b/drivers/vfio/pci/nvgrace-gpu/main.c
> @@ -561,7 +561,7 @@ nvgrace_gpu_map_and_read(struct nvgrace_gpu_pci_core_device *nvdev,
>   		ret = vfio_pci_core_do_io_rw(&nvdev->core_device, false,
>   					     nvdev->resmem.ioaddr,
>   					     buf, offset, mem_count,
> -					     0, 0, false);
> +					     0, 0, false, VFIO_PCI_IO_WIDTH_8);
>   	}
>   
>   	return ret;
> @@ -693,7 +693,7 @@ nvgrace_gpu_map_and_write(struct nvgrace_gpu_pci_core_device *nvdev,
>   		ret = vfio_pci_core_do_io_rw(&nvdev->core_device, false,
>   					     nvdev->resmem.ioaddr,
>   					     (char __user *)buf, pos, mem_count,
> -					     0, 0, true);
> +					     0, 0, true, VFIO_PCI_IO_WIDTH_8);
>   	}
>   
>   	return ret;
> diff --git a/drivers/vfio/pci/vfio_pci_rdwr.c b/drivers/vfio/pci/vfio_pci_rdwr.c
> index 6192788c8ba3..25380b7dfe18 100644
> --- a/drivers/vfio/pci/vfio_pci_rdwr.c
> +++ b/drivers/vfio/pci/vfio_pci_rdwr.c
> @@ -135,7 +135,8 @@ VFIO_IORDWR(64)
>   ssize_t vfio_pci_core_do_io_rw(struct vfio_pci_core_device *vdev, bool test_mem,
>   			       void __iomem *io, char __user *buf,
>   			       loff_t off, size_t count, size_t x_start,
> -			       size_t x_end, bool iswrite)
> +			       size_t x_end, bool iswrite,
> +			       enum vfio_pci_io_width max_width)
>   {
>   	ssize_t done = 0;
>   	int ret;
> @@ -150,20 +151,19 @@ ssize_t vfio_pci_core_do_io_rw(struct vfio_pci_core_device *vdev, bool test_mem,
>   		else
>   			fillable = 0;
>   
> -		if (fillable >= 8 && !(off % 8)) {
> +		if (fillable >= 8 && !(off % 8) && max_width >= 8) {
>   			ret = vfio_pci_iordwr64(vdev, iswrite, test_mem,
>   						io, buf, off, &filled);
>   			if (ret)
>   				return ret;
>   
> -		} else
> -		if (fillable >= 4 && !(off % 4)) {
> +		} else if (fillable >= 4 && !(off % 4) && max_width >= 4) {
>   			ret = vfio_pci_iordwr32(vdev, iswrite, test_mem,
>   						io, buf, off, &filled);
>   			if (ret)
>   				return ret;
>   
> -		} else if (fillable >= 2 && !(off % 2)) {
> +		} else if (fillable >= 2 && !(off % 2) && max_width >= 2) {
>   			ret = vfio_pci_iordwr16(vdev, iswrite, test_mem,
>   						io, buf, off, &filled);
>   			if (ret)
> @@ -234,6 +234,7 @@ ssize_t vfio_pci_bar_rw(struct vfio_pci_core_device *vdev, char __user *buf,
>   	void __iomem *io;
>   	struct resource *res = &vdev->pdev->resource[bar];
>   	ssize_t done;
> +	enum vfio_pci_io_width max_width = VFIO_PCI_IO_WIDTH_8;
>   
>   	if (pci_resource_start(pdev, bar))
>   		end = pci_resource_len(pdev, bar);
> @@ -262,6 +263,16 @@ ssize_t vfio_pci_bar_rw(struct vfio_pci_core_device *vdev, char __user *buf,
>   		if (!io)
>   			return -ENOMEM;
>   		x_end = end;
> +
> +		/*
> +		 * Certain devices (e.g. Intel X710) don't support qword
> +		 * access to the ROM bar. Otherwise PCI AER errors might be
> +		 * triggered.
> +		 *
> +		 * Disable qword access to the ROM bar universally, which
> +		 * worked reliably for years before qword access is enabled.
> +		 */
> +		max_width = VFIO_PCI_IO_WIDTH_4;
>   	} else {
>   		int ret = vfio_pci_core_setup_barmap(vdev, bar);
>   		if (ret) {
> @@ -278,7 +289,7 @@ ssize_t vfio_pci_bar_rw(struct vfio_pci_core_device *vdev, char __user *buf,
>   	}
>   
>   	done = vfio_pci_core_do_io_rw(vdev, res->flags & IORESOURCE_MEM, io, buf, pos,
> -				      count, x_start, x_end, iswrite);
> +				      count, x_start, x_end, iswrite, max_width);
>   
>   	if (done >= 0)
>   		*ppos += done;
> @@ -352,7 +363,7 @@ ssize_t vfio_pci_vga_rw(struct vfio_pci_core_device *vdev, char __user *buf,
>   	 * to the memory enable bit in the command register.
>   	 */
>   	done = vfio_pci_core_do_io_rw(vdev, false, iomem, buf, off, count,
> -				      0, 0, iswrite);
> +				      0, 0, iswrite, VFIO_PCI_IO_WIDTH_8);
>   
>   	vga_put(vdev->pdev, rsrc);
>   
> diff --git a/include/linux/vfio_pci_core.h b/include/linux/vfio_pci_core.h
> index 706877f998ff..1ac86896875c 100644
> --- a/include/linux/vfio_pci_core.h
> +++ b/include/linux/vfio_pci_core.h
> @@ -145,6 +145,13 @@ struct vfio_pci_core_device {
>   	struct list_head	dmabufs;
>   };
>   
> +enum vfio_pci_io_width {
> +	VFIO_PCI_IO_WIDTH_1 = 1,
> +	VFIO_PCI_IO_WIDTH_2 = 2,
> +	VFIO_PCI_IO_WIDTH_4 = 4,
> +	VFIO_PCI_IO_WIDTH_8 = 8,
> +};
> +
>   /* Will be exported for vfio pci drivers usage */
>   int vfio_pci_core_register_dev_region(struct vfio_pci_core_device *vdev,
>   				      unsigned int type, unsigned int subtype,
> @@ -188,7 +195,8 @@ pci_ers_result_t vfio_pci_core_aer_err_detected(struct pci_dev *pdev,
>   ssize_t vfio_pci_core_do_io_rw(struct vfio_pci_core_device *vdev, bool test_mem,
>   			       void __iomem *io, char __user *buf,
>   			       loff_t off, size_t count, size_t x_start,
> -			       size_t x_end, bool iswrite);
> +			       size_t x_end, bool iswrite,
> +			       enum vfio_pci_io_width max_width);
>   bool __vfio_pci_memory_enabled(struct vfio_pci_core_device *vdev);
>   bool vfio_pci_core_range_intersect_range(loff_t buf_start, size_t buf_cnt,
>   					 loff_t reg_start, size_t reg_cnt,
> -- 
> 2.43.0
> 

Tested-by: Farrah Chen <farrah.chen@intel.com>

With this patch, I tested device passthrough with an Intel X710 NIC. No 
PCIe errors were found, and the device works well in the guest.

[PATCH v2 2/2] vfio/pci: Disable qword access to the VGA region

[Kevin Tian]

Seems no reason to allow qword access to the old VGA resource. Better
restrict it to dword access as before.

Suggested-by: Alex Williamson <alex@shazbot.org>
Signed-off-by: Kevin Tian <kevin.tian@intel.com>
---
 drivers/vfio/pci/vfio_pci_rdwr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/vfio/pci/vfio_pci_rdwr.c b/drivers/vfio/pci/vfio_pci_rdwr.c
index 25380b7dfe18..b38627b35c35 100644
--- a/drivers/vfio/pci/vfio_pci_rdwr.c
+++ b/drivers/vfio/pci/vfio_pci_rdwr.c
@@ -363,7 +363,7 @@ ssize_t vfio_pci_vga_rw(struct vfio_pci_core_device *vdev, char __user *buf,
 	 * to the memory enable bit in the command register.
 	 */
 	done = vfio_pci_core_do_io_rw(vdev, false, iomem, buf, off, count,
-				      0, 0, iswrite, VFIO_PCI_IO_WIDTH_8);
+				      0, 0, iswrite, VFIO_PCI_IO_WIDTH_4);
 
 	vga_put(vdev->pdev, rsrc);
 
-- 
2.43.0

Re: [PATCH v2 0/2] vfio/pci: Fix issues with qword access

[Alex Williamson]

On Thu, 18 Dec 2025 08:16:48 +0000
Kevin Tian <kevin.tian@intel.com> wrote:

> Certain devices (e.g. Intel X710) don't support qword access to the
> rom bar, otherwise PCI aer errors are observed. Fix it by disabling
> the qword access to the rom bar.
> 
> While at it, also restrict accesses to the legacy VGA resource to
> dword. More for paranoia so stable kernel is not CC-ed.
> 
> v2:
> - Rebase to 6.19-rc1
> - Use enum to avoid adding another bool arg (Alex)
> - Elaborate the commit msg (Alex)
> - New patch to disallow qword access to legacy VGA (Alex)
> 
> v1:
> https://lore.kernel.org/all/20251212020941.338355-1-kevin.tian@intel.com/
> 
> Kevin Tian (2):
>   vfio/pci: Disable qword access to the PCI ROM bar
>   vfio/pci: Disable qword access to the VGA region
> 
>  drivers/vfio/pci/nvgrace-gpu/main.c |  4 ++--
>  drivers/vfio/pci/vfio_pci_rdwr.c    | 25 ++++++++++++++++++-------
>  include/linux/vfio_pci_core.h       | 10 +++++++++-
>  3 files changed, 29 insertions(+), 10 deletions(-)
> 

Applied to vfio for-linus branch for v6.19.  Thanks,

Alex