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Subject: Re: [RFC PATCH 6/7] iommu/amd: Introduce amd_iommu_pgtable
 command-line option
To:     Joerg Roedel <joro@8bytes.org>
Cc:     linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
        Jon.Grimm@amd.com, Wei.Huang2@amd.com
References: <20210312090411.6030-1-suravee.suthikulpanit@amd.com>
 <20210312090411.6030-7-suravee.suthikulpanit@amd.com>
 <YFNy0LbQAreu64lt@8bytes.org>
From:   Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
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Joerg,

On 3/18/21 10:33 PM, Joerg Roedel wrote:
> On Fri, Mar 12, 2021 at 03:04:10AM -0600, Suravee Suthikulpanit wrote:
>> To allow specification whether to use v1 or v2 IOMMU pagetable for
>> DMA remapping when calling kernel DMA-API.
>>
>> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
>> ---
>>   Documentation/admin-guide/kernel-parameters.txt |  6 ++++++
>>   drivers/iommu/amd/init.c                        | 15 +++++++++++++++
>>   2 files changed, 21 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index 04545725f187..466e807369ea 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -319,6 +319,12 @@
>>   			             This mode requires kvm-amd.avic=1.
>>   			             (Default when IOMMU HW support is present.)
>>   
>> +	amd_iommu_pgtable= [HW,X86-64]
>> +			Specifies one of the following AMD IOMMU page table to
>> +			be used for DMA remapping for DMA-API:
>> +			v1         - Use v1 page table (Default)
>> +			v2         - Use v2 page table
> 
> Any reason v2 can not be the default when it is supported by the IOMMU?
> 

Eventually, we should be able to default to v2. However, we will need to make sure that
the v2 implementation will have comparable performance as currently used v1.

FYI: I'm also looking into adding support for SVA as well.

Thanks,
Suravee