Re: [PATCH 2/6] iommu/amd: Clear DTE with update_dte256 in iommu_ignore_device()

[Jason Gunthorpe]

On Mon, Jun 01, 2026 at 01:42:00PM +0000, Pranjal Shrivastava wrote:
> The iommu_ignore_device() function currently uses memset() to clear
> the Device Table Entry (DTE), which risks torn writes because the
> hardware reads DTEs as atomic 256-bit qwords. Fix this by using
> update_dte256() to perform a hardware-safe atomic clear when a live
> dev_data entry is available.
> 
> Fixes: 99fc4ac3d297 ("iommu/amd: Introduce per PCI segment alias_table")
> Reported-by: sashiko-bot@xxxxxxxxxx
> Closes: https://lore.kernel.org/all/20260529153216.2AD1E1F00899@xxxxxxxxxxxxxxx/
> Signed-off-by: Pranjal Shrivastava <praan@xxxxxxxxxx>
> ---
>  drivers/iommu/amd/iommu.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
> index a94de66a885e..9b5861e241d7 100644
> --- a/drivers/iommu/amd/iommu.c
> +++ b/drivers/iommu/amd/iommu.c
> @@ -750,7 +750,16 @@ static void iommu_ignore_device(struct amd_iommu *iommu, struct device *dev)
>  	setup_aliases(iommu, dev);
>  
>  	pci_seg->rlookup_table[devid] = NULL;
> -	memset(&dev_table[devid], 0, sizeof(struct dev_table_entry));
> +
> +	/* Clear DTE if we have a live entry */
> +	if (dev_data) {
> +		struct dev_table_entry new = {};
> +
> +		amd_iommu_make_clear_dte(dev_data, &new);
> +		update_dte256(iommu, dev_data, &new);
> +	} else {
> +		memset(&dev_table[devid], 0, sizeof(struct dev_table_entry));
> +	}

This seems a little weird, an ignored device shouldn't have a dev_data
really, or it will soon be freed.

I think you are better to replace the memset with a dedicated function

/* Cannot not be used on a probe'd device with a live dev_data */
disable_dte(..)
{
	struct dev_table_entry new = {};

	write_dte_lower128(ptr, new);
	write_dte_upper128(ptr, new);
}

And then this new ordering breaks the clone_aliases flow, it was
supposed to copy the 0 from the current DTE to the aliases..

Jason