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Re: [XEN PATCH 2/3] AMD-Vi: Disable IOMMU if cx16 isn't supported


  • To: Teddy Astie <teddy.astie@xxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Thu, 21 Mar 2024 11:41:13 +0100
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  • Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Thu, 21 Mar 2024 10:41:25 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 21.03.2024 11:28, Teddy Astie wrote:
> No hardware has VT-d support while not having cx16 support, consider 
> disabling IOMMU in this case to avoid potentially buggy code.

VT-d? That's Intel, not AMD. Also alongside bare hardware you also want to
not completely leave out Xen running virtualized itself.

As a nit, please limit description lines to 80 chars.

> --- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
> +++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
> @@ -312,6 +312,12 @@ static int __init cf_check iov_detect(void)
>          return -ENODEV;
>      }
>  
> +    if ( unlikely(!cpu_has_cx16) )
> +    {
> +        printk("AMD-Vi: CPU doesn't support CMPXCHG16B, disabling\n");
> +        return -ENOSYS;

ENOSYS has a dedicated purpose and hence doesn't fit here. If no better
suitable error code can be identified, ENODEV is perhaps what ought to be
used.

I'd also like to note that while this says 2/3, I can't spot any other
part of such a 3-patch series anywhere. However, in particular
https://lists.xen.org/archives/html/xen-devel/2024-03/threads.html looks
to be lagging anyway.

Further please note that maintainership recently has changed some; please
make sure you use up-to-date information for composing the Cc: list.

Jan



 


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