Re: [PATCH v2 06/12] VT-d: respect ACPI SATC's ATC_REQUIRED flag

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Feb 15, 2024 at 11:16:11AM +0100, Jan Beulich wrote:
> When the flag is set, permit Dom0 to control the device (no worse than
> what we had before and in line with other "best effort" behavior we use
> when it comes to Dom0),

I think we should somehow be able to signal dom0 that this device
might not operate as expected, otherwise dom0 might use it and the
device could silently malfunction due to ATS not being enabled.

Otherwise we should just hide the device from dom0.

I assume setting the IOMMU context entry to passthrough mode would
also be fine for such devices that require ATS?

> but suppress passing through to DomU-s unless
> ATS can actually be enabled for such devices (and was explicitly enabled
> on the command line).
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> v2: Re-base over new earlier patches.
> 
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -225,7 +225,11 @@ exceptions (watchdog NMIs and unexpected
>  > Default: `false`
>  
>  Permits Xen to set up and use PCI Address Translation Services.  This is a
> -performance optimisation for PCI Passthrough.
> +performance optimisation for PCI Passthrough.  Note that firmware may indicate
> +that certain devices need to have ATS enabled for proper operation. For such
> +devices ATS will be enabled by default, unless the option is used in its
> +negative form.  Such devices will still not be eligible for passing through to
> +guests, unless the option is used in its positive form.
>  
>  **WARNING: Xen cannot currently safely use ATS because of its synchronous wait
>  loops for Queued Invalidation completions.**
> --- a/xen/drivers/passthrough/vtd/dmar.c
> +++ b/xen/drivers/passthrough/vtd/dmar.c
> @@ -253,6 +253,24 @@ struct acpi_atsr_unit *acpi_find_matched
>      return all_ports;
>  }
>  
> +const struct acpi_satc_unit *acpi_find_matched_satc_unit(
> +    const struct pci_dev *pdev)
> +{
> +    const struct acpi_satc_unit *satc;
> +
> +    list_for_each_entry ( satc, &acpi_satc_units, list )
> +    {
> +        if ( satc->segment != pdev->seg )
> +            continue;
> +
> +        for ( unsigned int i = 0; i < satc->scope.devices_cnt; ++i )
> +            if ( satc->scope.devices[i] == pdev->sbdf.bdf )
> +                return satc;
> +    }
> +
> +    return NULL;
> +}
> +
>  struct acpi_rhsa_unit *drhd_to_rhsa(const struct acpi_drhd_unit *drhd)
>  {
>      struct acpi_rhsa_unit *rhsa;
> --- a/xen/drivers/passthrough/vtd/dmar.h
> +++ b/xen/drivers/passthrough/vtd/dmar.h
> @@ -112,6 +112,8 @@ struct acpi_satc_unit {
>  
>  struct acpi_drhd_unit *acpi_find_matched_drhd_unit(const struct pci_dev *);
>  struct acpi_atsr_unit *acpi_find_matched_atsr_unit(const struct pci_dev *);
> +const struct acpi_satc_unit *acpi_find_matched_satc_unit(
> +    const struct pci_dev *pdev);
>  
>  #define DMAR_TYPE 1
>  #define RMRR_TYPE 2
> --- a/xen/drivers/passthrough/vtd/iommu.c
> +++ b/xen/drivers/passthrough/vtd/iommu.c
> @@ -2364,6 +2364,26 @@ static int cf_check intel_iommu_add_devi
>      if ( ret )
>          dprintk(XENLOG_ERR VTDPREFIX, "%pd: context mapping failed\n",
>                  pdev->domain);
> +    else if ( !pdev->broken )
> +    {
> +        const struct acpi_drhd_unit *drhd = acpi_find_matched_drhd_unit(pdev);
> +        const struct acpi_satc_unit *satc = acpi_find_matched_satc_unit(pdev);
> +
> +        /*
> +         * Prevent the device from getting assigned to an unprivileged domain
> +         * when firmware indicates ATS is required, but ATS could not be enabled
> +         * or was not explicitly enabled via command line option.
> +         */
> +        if ( satc && satc->atc_required &&
> +             (!drhd || ats_device(pdev, drhd) <= 0 ||
> +              !pci_ats_enabled(pdev->seg, pdev->bus, pdev->devfn) ||
> +              opt_ats < 0) )

Do you need the opt_ats check here?

I don't think it's possible for pci_ats_enabled() to return true if
opt_ats is <= 0, and hence the opt_ats < 0 check can be dropped from
the conditional?

> +        {
> +            printk(XENLOG_WARNING "ATS: %pp is not eligible for pass-through\n",
> +                   &pdev->sbdf);
> +            pdev->broken = true;
> +        }
> +    }
>  
>      return ret;
>  }
> @@ -2375,12 +2395,26 @@ static int cf_check intel_iommu_enable_d
>  
>      pci_vtd_quirk(pdev);
>  
> -    if ( ret <= 0 )
> -        return ret;
> +    if ( ret <= 0 ||
> +         (ret = enable_ats_device(pdev, &drhd->iommu->ats_devices)) < 0 ||
> +         opt_ats < 0 )

Shouldn't this be opt_ats <= 0?

> +    {
> +        const struct acpi_satc_unit *satc = acpi_find_matched_satc_unit(pdev);
> +
> +        /*
> +         * Besides in error cases also prevent the device from getting assigned
> +         * to an unprivileged domain when firmware indicates ATS is required,
> +         * but ATS use was not explicitly enabled via command line option.
> +         */
> +        if ( satc && satc->atc_required && !pdev->broken )
> +        {
> +            printk(XENLOG_WARNING "ATS: %pp is not eligible for pass-through\n",
> +                   &pdev->sbdf);
> +            pdev->broken = true;
> +        }

I think the code here could be easier to read if this was put in an
label at the end, and the early return above that you remove becomes a
goto.  But that's a question of taste.

> +    }
>  
> -    ret = enable_ats_device(pdev, &drhd->iommu->ats_devices);
> -
> -    return ret >= 0 ? 0 : ret;
> +    return ret <= 0 ? ret : 0;
>  }
>  
>  static int cf_check intel_iommu_remove_device(u8 devfn, struct pci_dev *pdev)
> --- a/xen/drivers/passthrough/vtd/x86/ats.c
> +++ b/xen/drivers/passthrough/vtd/x86/ats.c
> @@ -45,8 +45,9 @@ int ats_device(const struct pci_dev *pde
>  {
>      struct acpi_drhd_unit *ats_drhd;
>      unsigned int pos, expfl = 0;
> +    const struct acpi_satc_unit *satc;
>  
> -    if ( opt_ats <= 0 || !iommu_qinval )
> +    if ( !opt_ats || !iommu_qinval )
>          return 0;

FWIW, I find this change confusing, hence my request earlier that
opt_ats must be set to 0 or 1 by the point it gets used.

Thanks, Roger.