linux-kernel - Re: [PATCH v14 6/6] iommu: Remove mode argument from iommu_set_dma

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Date:   Mon, 21 Jun 2021 18:00:25 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     John Garry <john.garry@...wei.com>, joro@...tes.org,
        will@...nel.org, dwmw2@...radead.org, robin.murphy@....com,
        corbet@....net
Cc:     baolu.lu@...ux.intel.com, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linuxarm@...wei.com,
        thunder.leizhen@...wei.com, chenxiang66@...ilicon.com,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v14 6/6] iommu: Remove mode argument from
 iommu_set_dma_strict()

On 2021/6/21 16:12, John Garry wrote:
> On 21/06/2021 06:17, Lu Baolu wrote:
>> On 2021/6/18 19:34, John Garry wrote:
>>> 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;
>>>   }
>>
> 
> Hi baolu,

Hi John,

> 
>> Sorry for this late comment.
>>  > Normally the cache invalidation policy should come from the user. We
>> have pre-build kernel option and also a kernel boot command iommu.strict
>> to override it. These seem reasonable.
>>
>> We also have a helper (iommu_set_dma_strict()) so that the vendor iommu
>> driver could squeeze in and change the previous settings mostly due to:
>>
>> a) vendor iommu driver specific kernel boot command. (We are about to
>>     deprecate those.)
>>
>> b) quirky hardware.
>>
>> c) kernel optimization (e.x. strict mode in VM environment).
>>
>> a) and b) are mandatory, while c) is optional. In any instance should c)
>> override the flush mode specified by the user. Hence, probably we should
>> also have another helper like:
>>
>> void iommu_set_dma_strict_optional()
>> {
>>      if (!(iommu_cmd_line & IOMMU_CMD_LINE_STRICT))
>>          iommu_dma_strict = true;
>> }
>>
>> Any thoughts?
> 
> What you are suggesting is a change in policy from mainline code. 
> Currently for c) we always set strict enabled, regardless of any user 
> cmdline input. But now you are saying that you want iommu.strict to 
> override in particular scenario, right?
> 
> In that case I would think it's better to rework the current API, like 
> adding an option to "force" strict mode:
> 
> void iommu_set_dma_strict(bool force)
> {
>           if (force == true)
>          iommu_dma_strict = true;
>      else if (!(iommu_cmd_line & IOMMU_CMD_LINE_STRICT))
>          iommu_dma_strict = true;
> }
> 
> So we would use iommu_set_dma_strict(true) for a) and b), but 
> iommu_set_dma_strict(false) for c).

Yes. We need to distinguish the "must" and "nice-to-have" cases of
setting strict mode.

> 
> Then I am not sure what you want to do with the accompanying print for 
> c). It was:
> "IOMMU batching is disabled due to virtualization"
> 
> And now is from this series:
> "IOMMU batching disallowed due to virtualization"
> 
> Using iommu_get_dma_strict(domain) is not appropriate here to know the 
> current mode (so we know whether to print).
> 
> Note that this change would mean that the current series would require 
> non-trivial rework, which would be unfortunate so late in the cycle.

This patch series looks good to me and I have added by reviewed-by.
Probably we could make another patch series to improve it so that the
kernel optimization should not override the user setting.

Best regards,
baolu