RE: [PATCH v2 4/6] iommu/vt-d: Remove 1:1 mappings from identity domain

Tian, Kevin
2023-12-13T03:04:20Z

> From: Baolu Lu <baolu.lu@linux.intel.com>
> Sent: Wednesday, December 13, 2023 10:44 AM
>
> On 12/13/23 10:20 AM, Tian, Kevin wrote:
> >>> if (!dmar_map_gfx)
> >>> iommu_identity_mapping |= IDENTMAP_GFX;
> >> So with above cleaned up, we have no need to worry about drivers that
> >> are not capable of handling remapped dma address any more.
> >>
> >> Did I miss anything?
> > I prefer to removing IDENTMAP_GFX in this series and put a comment
> > explaining why Azalia device has no problem.
> >
> > Then send a separate patch to remove the GFX workaround option.
> > If there is any valid usage still relying on that, it's easy to revert.
>
> Agreed. We should be more cautious. Perhaps I will postpone this series
> to a time when we are sure that graphic drivers are okay with this
> change. As a first step, perhaps we can make a change to remove the
> workaround for graphic drivers, so that any hidden bugs in the graphic
> driver could be reported.
>
> The patch looks like,
>
> iommu/vt-d: Remove INTEL_IOMMU_BROKEN_GFX_WA
>
> Commit 62edf5dc4a524 ("intel-iommu: Restore DMAR_BROKEN_GFX_WA
> option for
> broken graphics drivers") was introduced 24 years ago as a temporary
> workaround for graphics drivers that used physical addresses for DMA and
> avoided DMA APIs. This workaround was disabled by default.
>
> As 24 years have passed, it is expected that graphics driver developers
> have migrated their drivers to use kernel DMA APIs. Therefore, this
> workaround is no longer required and could been removed.

and igfx_off option is still available just in case.

>
> Suggested-by: Kevin Tian <kevin.tian@intel.com>
> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
> ---
> drivers/iommu/intel/iommu.c | 10 ----------
> drivers/iommu/intel/Kconfig | 11 -----------
> 2 files changed, 21 deletions(-)
>
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 84b78e42a470..27b8638291f2 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -2357,9 +2357,6 @@ static int device_def_domain_type(struct device
> *dev)
>
> if ((iommu_identity_mapping & IDENTMAP_AZALIA) &&
> IS_AZALIA(pdev))
> return IOMMU_DOMAIN_IDENTITY;
> -
> - if ((iommu_identity_mapping & IDENTMAP_GFX) &&
> IS_GFX_DEVICE(pdev))
> - return IOMMU_DOMAIN_IDENTITY;
> }
>
> return 0;
> @@ -2660,13 +2657,6 @@ static int __init init_dmars(void)
> iommu_set_root_entry(iommu);
> }
>
> -#ifdef CONFIG_INTEL_IOMMU_BROKEN_GFX_WA
> - dmar_map_gfx = 0;
> -#endif
> -
> - if (!dmar_map_gfx)
> - iommu_identity_mapping |= IDENTMAP_GFX;
> -

let's remove IDENTMAP_GFX (and all its references) in a separate patch.

this patch is for removing the workaround option.

another patch removes IDENTMAP_GFX as it's meaningless when
dmar_map_gfx simply implies disabling the gfx iommu.

> check_tylersburg_isoch();
>
> ret = si_domain_init(hw_pass_through);
> diff --git a/drivers/iommu/intel/Kconfig b/drivers/iommu/intel/Kconfig
> index 012cd2541a68..d2d34eb28d94 100644
> --- a/drivers/iommu/intel/Kconfig
> +++ b/drivers/iommu/intel/Kconfig
> @@ -64,17 +64,6 @@ config INTEL_IOMMU_DEFAULT_ON
> one is found. If this option is not selected, DMAR support can
> be enabled by passing intel_iommu=on to the kernel.
>
> -config INTEL_IOMMU_BROKEN_GFX_WA
> - bool "Workaround broken graphics drivers (going away soon)"
> - depends on BROKEN && X86
> - help
> - Current Graphics drivers tend to use physical address
> - for DMA and avoid using DMA APIs. Setting this config
> - option permits the IOMMU driver to set a unity map for
> - all the OS-visible memory. Hence the driver can continue
> - to use physical addresses for DMA, at least until this
> - option is removed in the 2.6.32 kernel.
> -
> config INTEL_IOMMU_FLOPPY_WA
> def_bool y
> depends on X86
>
> Best regards,
> baolu