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Subject: Re: [PATCH v13 6/6] iommu: Remove mode argument from
 iommu_set_dma_strict()
To:     Lu Baolu <baolu.lu@linux.intel.com>,
        John Garry <john.garry@huawei.com>, joro@8bytes.org,
        will@kernel.org, dwmw2@infradead.org, corbet@lwn.net
Cc:     linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
        linuxarm@huawei.com, thunder.leizhen@huawei.com,
        chenxiang66@hisilicon.com, linux-doc@vger.kernel.org
References: <1623841437-211832-1-git-send-email-john.garry@huawei.com>
 <1623841437-211832-7-git-send-email-john.garry@huawei.com>
 <de6a2874-3d6d-ed2a-78f5-fb1fb0195228@linux.intel.com>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <7d0fb0e2-4671-16db-6963-b0493d7a549b@arm.com>
Date:   Thu, 17 Jun 2021 19:56:44 +0100
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On 2021-06-17 08:36, Lu Baolu wrote:
> On 6/16/21 7:03 PM, John Garry wrote:
>> We only ever now set strict mode enabled in iommu_set_dma_strict(), so
>> just remove the argument.
>>
>> Signed-off-by: John Garry <john.garry@huawei.com>
>> Reviewed-by: Robin Murphy <robin.murphy@arm.com>
>> ---
>>   drivers/iommu/amd/init.c    | 2 +-
>>   drivers/iommu/intel/iommu.c | 6 +++---
>>   drivers/iommu/iommu.c       | 5 ++---
>>   include/linux/iommu.h       | 2 +-
>>   4 files changed, 7 insertions(+), 8 deletions(-)
>>
>> diff --git a/drivers/iommu/amd/init.c b/drivers/iommu/amd/init.c
>> index fb3618af643b..7bc460052678 100644
>> --- a/drivers/iommu/amd/init.c
>> +++ b/drivers/iommu/amd/init.c
>> @@ -3099,7 +3099,7 @@ static int __init parse_amd_iommu_options(char 
>> *str)
>>       for (; *str; ++str) {
>>           if (strncmp(str, "fullflush", 9) == 0) {
>>               pr_warn("amd_iommu=fullflush deprecated; use 
>> iommu.strict instead\n");
>> -            iommu_set_dma_strict(true);
>> +            iommu_set_dma_strict();
>>           }
>>           if (strncmp(str, "force_enable", 12) == 0)
>>               amd_iommu_force_enable = true;
>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>> index d586990fa751..0618c35cfb51 100644
>> --- a/drivers/iommu/intel/iommu.c
>> +++ b/drivers/iommu/intel/iommu.c
>> @@ -454,7 +454,7 @@ static int __init intel_iommu_setup(char *str)
>>               iommu_dma_forcedac = true;
>>           } else if (!strncmp(str, "strict", 6)) {
>>               pr_warn("intel_iommu=strict deprecated; use iommu.strict 
>> instead\n");
>> -            iommu_set_dma_strict(true);
>> +            iommu_set_dma_strict();
>>           } else if (!strncmp(str, "sp_off", 6)) {
>>               pr_info("Disable supported super page\n");
>>               intel_iommu_superpage = 0;
>> @@ -4382,7 +4382,7 @@ int __init intel_iommu_init(void)
>>            */
>>           if (cap_caching_mode(iommu->cap)) {
>>               pr_warn("IOMMU batching disallowed due to 
>> virtualization\n");
>> -            iommu_set_dma_strict(true);
>> +            iommu_set_dma_strict();
>>           }
>>           iommu_device_sysfs_add(&iommu->iommu, NULL,
>>                          intel_iommu_groups,
>> @@ -5699,7 +5699,7 @@ static void quirk_calpella_no_shadow_gtt(struct 
>> pci_dev *dev)
>>       } else if (dmar_map_gfx) {
>>           /* we have to ensure the gfx device is idle before we flush */
>>           pci_info(dev, "Disabling batched IOTLB flush on Ironlake\n");
>> -        iommu_set_dma_strict(true);
>> +        iommu_set_dma_strict();
>>       }
>>   }
>>   DECLARE_PCI_FIXUP_HEADER(PCI_VENDOR_ID_INTEL, 0x0040, 
>> quirk_calpella_no_shadow_gtt);
>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>> index 60b1ec42e73b..ff221d3ddcbc 100644
>> --- a/drivers/iommu/iommu.c
>> +++ b/drivers/iommu/iommu.c
>> @@ -349,10 +349,9 @@ static int __init iommu_dma_setup(char *str)
>>   }
>>   early_param("iommu.strict", iommu_dma_setup);
>> -void iommu_set_dma_strict(bool strict)
>> +void iommu_set_dma_strict(void)
>>   {
>> -    if (strict || !(iommu_cmd_line & IOMMU_CMD_LINE_STRICT))
>> -        iommu_dma_strict = strict;
>> +    iommu_dma_strict = true;
> 
> Sorry, I still can't get how iommu.strict kernel option works.
> 
> static int __init iommu_dma_setup(char *str)
> {
>          int ret = kstrtobool(str, &iommu_dma_strict);

Note that this is the bit that does the real work - if the argument 
parses OK then iommu_dma_strict is reassigned with the appropriate 
value. The iommu_cmd_line stuff is a bit of additional bookkeeping, 
basically just so we can see whether default values have been overridden.

Robin.

> 
>          if (!ret)
>                  iommu_cmd_line |= IOMMU_CMD_LINE_STRICT;
>          return ret;
> }
> early_param("iommu.strict", iommu_dma_setup);
> 
> The bit IOMMU_CMD_LINE_STRICT is only set, but not used anywhere. Hence,
> I am wondering how could it work? A bug or I missed anything?
> 
> Best regards,
> baolu
> 
>>   }
>>   bool iommu_get_dma_strict(struct iommu_domain *domain)
>> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
>> index 32d448050bf7..754f67d6dd90 100644
>> --- a/include/linux/iommu.h
>> +++ b/include/linux/iommu.h
>> @@ -476,7 +476,7 @@ int iommu_enable_nesting(struct iommu_domain 
>> *domain);
>>   int iommu_set_pgtable_quirks(struct iommu_domain *domain,
>>           unsigned long quirks);
>> -void iommu_set_dma_strict(bool val);
>> +void iommu_set_dma_strict(void);
>>   bool iommu_get_dma_strict(struct iommu_domain *domain);
>>   extern int report_iommu_fault(struct iommu_domain *domain, struct 
>> device *dev,
>>