[PATCH] iommu: Print default strict or lazy mode at init time

[PATCH] iommu: Print default strict or lazy mode at init time

[John Garry]

As well as the default domain type, it's useful to know whether strict
or lazy mode is default for DMA domains, so add this info in a separate
print.

Signed-off-by: John Garry <john.garry@huawei.com>

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 808ab70d5df5..f25fae62f077 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -138,6 +138,11 @@ static int __init iommu_subsys_init(void)
 		(iommu_cmd_line & IOMMU_CMD_LINE_DMA_API) ?
 			"(set via kernel command line)" : "");
 
+	pr_info("Default DMA domain mode: %s %s\n",
+		iommu_dma_strict ? "strict" : "lazy",
+		(iommu_cmd_line & IOMMU_CMD_LINE_STRICT) ?
+			"(set via kernel command line)" : "");
+
 	return 0;
 }
 subsys_initcall(iommu_subsys_init);
-- 
2.26.2

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Re: [PATCH] iommu: Print default strict or lazy mode at init time

[Robin Murphy]

On 2021-05-28 14:37, John Garry wrote:
> As well as the default domain type, it's useful to know whether strict
> or lazy mode is default for DMA domains, so add this info in a separate
> print.
> 
> Signed-off-by: John Garry <john.garry@huawei.com>
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 808ab70d5df5..f25fae62f077 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -138,6 +138,11 @@ static int __init iommu_subsys_init(void)
>   		(iommu_cmd_line & IOMMU_CMD_LINE_DMA_API) ?
>   			"(set via kernel command line)" : "");
>   
> +	pr_info("Default DMA domain mode: %s %s\n",

Nit: I think this might be a little unclear for end-users - *I'm* not 
even sure whether "Default" here is meant to refer to the mode setting 
itself or to default domains (of DMA type). Maybe something like "DMA 
domain TLB invalidation policy"? Certainly it seems like a good idea to 
explicitly mention invalidation to correlate with the documentation of 
the "iommu.strict" parameter.

Ack to the general idea though.

Thanks,
Robin.

> +		iommu_dma_strict ? "strict" : "lazy",
> +		(iommu_cmd_line & IOMMU_CMD_LINE_STRICT) ?
> +			"(set via kernel command line)" : "");
> +
>   	return 0;
>   }
>   subsys_initcall(iommu_subsys_init);
> 
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Re: [PATCH] iommu: Print default strict or lazy mode at init time

[John Garry]

On 01/06/2021 10:09, Robin Murphy wrote:
>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>> index 808ab70d5df5..f25fae62f077 100644
>> --- a/drivers/iommu/iommu.c
>> +++ b/drivers/iommu/iommu.c
>> @@ -138,6 +138,11 @@ static int __init iommu_subsys_init(void)
>>           (iommu_cmd_line & IOMMU_CMD_LINE_DMA_API) ?
>>               "(set via kernel command line)" : "");
>> +    pr_info("Default DMA domain mode: %s %s\n",
> 
> Nit: I think this might be a little unclear for end-users - *I'm* not 
> even sure whether "Default" here is meant to refer to the mode setting 
> itself or to default domains (of DMA type). Maybe something like "DMA 
> domain TLB invalidation policy"? Certainly it seems like a good idea to 
> explicitly mention invalidation to correlate with the documentation of 
> the "iommu.strict" parameter.
> 
> Ack to the general idea though.

ok, so I'll go with this:

pr_info("DMA domain default TLB invalidation policy: %s mode %s\n",
               iommu_dma_strict ? "strict" : "lazy",
                (iommu_cmd_line & IOMMU_CMD_LINE_STRICT) ?
                        "(set via kernel command line)" : "");

I think it's worth mentioning "default" somewhere, as not all IOMMUs or 
devices will use lazy mode even if it's default.

Thanks,
John
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Re: [PATCH] iommu: Print default strict or lazy mode at init time

[Robin Murphy]

On 2021-06-01 16:50, John Garry wrote:
> On 01/06/2021 10:09, Robin Murphy wrote:
>>> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
>>> index 808ab70d5df5..f25fae62f077 100644
>>> --- a/drivers/iommu/iommu.c
>>> +++ b/drivers/iommu/iommu.c
>>> @@ -138,6 +138,11 @@ static int __init iommu_subsys_init(void)
>>>           (iommu_cmd_line & IOMMU_CMD_LINE_DMA_API) ?
>>>               "(set via kernel command line)" : "");
>>> +    pr_info("Default DMA domain mode: %s %s\n",
>>
>> Nit: I think this might be a little unclear for end-users - *I'm* not 
>> even sure whether "Default" here is meant to refer to the mode setting 
>> itself or to default domains (of DMA type). Maybe something like "DMA 
>> domain TLB invalidation policy"? Certainly it seems like a good idea 
>> to explicitly mention invalidation to correlate with the documentation 
>> of the "iommu.strict" parameter.
>>
>> Ack to the general idea though.
> 
> ok, so I'll go with this:
> 
> pr_info("DMA domain default TLB invalidation policy: %s mode %s\n",
>                iommu_dma_strict ? "strict" : "lazy",
>                 (iommu_cmd_line & IOMMU_CMD_LINE_STRICT) ?
>                         "(set via kernel command line)" : "");
> 
> I think it's worth mentioning "default" somewhere, as not all IOMMUs or 
> devices will use lazy mode even if it's default.

But that's part of what I think is misleading - I boot and see that the 
default is something, so I reboot with iommu.strict to explicitly set it 
the other way, but now that's the default... huh?

The way I see it, we're saying what the current IOMMU API policy is - 
the value of iommu_dma_strict at any given time is fact - but we're not 
necessarily saying how widely that policy is enforced. We similarly 
report the type for default domains from global policy even though that 
may also be overridden per-group by drivers and/or userspace later; we 
don't say it's the *default* default domain type.

However, having now debugged the AMD issue from another thread, I think 
doing this at subsys_initcall is in fact going to be too early to be 
meaningful, since it ignores drivers' ability to change the global policy :(

Robin.
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Re: [PATCH] iommu: Print default strict or lazy mode at init time

[John Garry]

>>
>> pr_info("DMA domain default TLB invalidation policy: %s mode %s\n",
>>                iommu_dma_strict ? "strict" : "lazy",
>>                 (iommu_cmd_line & IOMMU_CMD_LINE_STRICT) ?
>>                         "(set via kernel command line)" : "");
>>
>> I think it's worth mentioning "default" somewhere, as not all IOMMUs 
>> or devices will use lazy mode even if it's default.
> 
> But that's part of what I think is misleading - I boot and see that the 
> default is something, so I reboot with iommu.strict to explicitly set it 
> the other way, but now that's the default... huh?
> 
> The way I see it, we're saying what the current IOMMU API policy is - 
> the value of iommu_dma_strict at any given time is fact - but we're not 
> necessarily saying how widely that policy is enforced. We similarly 
> report the type for default domains from global policy even though that 
> may also be overridden per-group by drivers and/or userspace later; 

> we 
> don't say it's the *default* default domain type.

I think that is this is the behavior a user would understand from that 
message.

However from a glance at the intel IOMMU driver, it seems possible to 
change default domain type after iommu_subsys_init().

> 
> However, having now debugged the AMD issue from another thread, I think 
> doing this at subsys_initcall is in fact going to be too early to be 
> meaningful, since it ignores drivers' ability to change the global 
> policy :(

A user may still learn the IOMMU group domain type from sysfs. There is 
no such thing for TLB invalidation mode - how about add a file for this? 
It would be useful.

Thanks,
John
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