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Re: [PATCH 4/5] vtd: Don't disable hwdom passthrough on unhandled SAGAW bits
- To: Teddy Astie <teddy.astie@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 5 Aug 2026 11:41:16 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 05 Aug 2026 09:41:20 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 29.07.2026 11:59, Teddy Astie wrote:
> On recent VT-d spec, bit 3 indicates support for 5-level pagetables,
> this is currently considered unhandled and causes "hwdom passthrough"
> to be disabled, even though it's unrelated.
>
> Given these are is capability bits, we don't need to consider unhandled
> bits, but only make sure that the ones we want (e.g 39-bit or 48-bit AGAW)
> are set.
>
> Fixes: 474fc7d3c652 ("iommu/vt-d: fix SAGAW capability parsing")
The description of that commit explains pretty well why pass-through mode
does need disabling in that case. If there's anything wrong with that
explanation, this would need calling out here.
Additionally I can only repeat my proposal to finally default to strict
mode. In strict mode, pass-through mode is disabled anyway. (IOW there's
the additional question of why you need pass-through mode in the first
place.)
> --- a/xen/drivers/passthrough/vtd/iommu.c
> +++ b/xen/drivers/passthrough/vtd/iommu.c
> @@ -1327,14 +1327,7 @@ int __init iommu_alloc(struct acpi_drhd_unit *drhd)
> }
>
> if ( sagaw >> 3 )
> - {
> - printk_once(XENLOG_WARNING VTDPREFIX
> - " Unhandled bits in SAGAW %#x%s\n",
> - sagaw,
> - iommu_hwdom_passthrough ? ", disabling passthrough" :
> "");
> -
> - iommu_hwdom_passthrough = false;
> - }
> + printk_once(XENLOG_WARNING VTDPREFIX " Unhandled bits in SAGAW
> %#x\n", sagaw);
Also please adhere to the line length limit.
Jan
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