Re: [PATCH v3 2/2] x86/iommu: avoid MSI address and data writes if IRT index hasn't changed

[Jan Beulich <jbeulich@suse.com>]

On 10.03.2025 10:55, Roger Pau Monne wrote:
> Attempt to reduce the MSI entry writes, and the associated checking whether
> memory decoding and MSI-X is enabled for the PCI device, when the MSI data
> hasn't changed.
> 
> When using Interrupt Remapping the MSI entry will contain an index into
> the remapping table, and it's in such remapping table where the MSI vector
> and destination CPU is stored.  As such, when using interrupt remapping,
> changes to the interrupt affinity shouldn't result in changes to the MSI
> entry, and the MSI entry update can be avoided.
> 
> Signal from the IOMMU update_ire_from_msi hook whether the MSI data or
> address fields have changed, and thus need writing to the device registers.
> Such signaling is done by returning 1 from the function.  Otherwise
> returning 0 means no update of the MSI fields, and thus no write
> required.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>

Reviewed-by: Jan Beulich <jbeulich@suse.com>
with two purely cosmetic suggestions and an only loosely related question below.

> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -415,7 +415,9 @@ static int cf_check vmx_pi_update_irte(const struct vcpu *v,
>  
>      ASSERT_PDEV_LIST_IS_READ_LOCKED(msi_desc->dev->domain);
>  
> -    return iommu_update_ire_from_msi(msi_desc, &msi_desc->msg);
> +    rc = iommu_update_ire_from_msi(msi_desc, &msi_desc->msg);
> +
> +    return rc < 0 ? rc : 0;

Only tangential here, but: Why does this function have a return type of
non-void, when neither caller cares?

> --- a/xen/drivers/passthrough/amd/iommu_intr.c
> +++ b/xen/drivers/passthrough/amd/iommu_intr.c
> @@ -492,7 +492,7 @@ static int update_intremap_entry_from_msi_msg(
>                 get_ivrs_mappings(iommu->seg)[alias_id].intremap_table);
>      }
>  
> -    return 0;
> +    return !fresh ? 0 : 1;
>  }

Simply

    return fresh;

?

> @@ -546,7 +546,7 @@ int cf_check amd_iommu_msi_msg_update_ire(
>      rc = update_intremap_entry_from_msi_msg(iommu, bdf, nr,
>                                              &msi_desc->remap_index,
>                                              msg, &data);
> -    if ( !rc )
> +    if ( rc > 0 )
>      {
>          for ( i = 1; i < nr; ++i )
>              msi_desc[i].remap_index = msi_desc->remap_index + i;
> --- a/xen/drivers/passthrough/vtd/intremap.c
> +++ b/xen/drivers/passthrough/vtd/intremap.c
> @@ -506,6 +506,7 @@ static int msi_msg_to_remap_entry(
>      unsigned int index, i, nr = 1;
>      unsigned long flags;
>      const struct pi_desc *pi_desc = msi_desc->pi_desc;
> +    bool alloc = false;
>  
>      if ( msi_desc->msi_attrib.type == PCI_CAP_ID_MSI )
>          nr = msi_desc->msi.nvec;
> @@ -529,6 +530,7 @@ static int msi_msg_to_remap_entry(
>          index = alloc_remap_entry(iommu, nr);
>          for ( i = 0; i < nr; ++i )
>              msi_desc[i].remap_index = index + i;
> +        alloc = true;
>      }
>      else
>          index = msi_desc->remap_index;
> @@ -601,7 +603,7 @@ static int msi_msg_to_remap_entry(
>      unmap_vtd_domain_page(iremap_entries);
>      spin_unlock_irqrestore(&iommu->intremap.lock, flags);
>  
> -    return 0;
> +    return alloc ? 1 : 0;
>  }

Like above, simply

    return alloc;

?

Jan