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

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Message-ID: <7d3a30cd-bd70-47e3-948e-32b0b68da3a3@linux.intel.com>
Date:   Wed, 13 Dec 2023 11:02:07 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...pe.ca>
Cc:     baolu.lu@...ux.intel.com,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/6] iommu/vt-d: Remove 1:1 mappings from identity
 domain

On 12/13/23 11:04 AM, Tian, Kevin wrote:
>> From: Baolu Lu<baolu.lu@...ux.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.

Yeah!

> 
>> Suggested-by: Kevin Tian<kevin.tian@...el.com>
>> Signed-off-by: Lu Baolu<baolu.lu@...ux.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.

Okay, it's fine to me.

Best regards,
baolu