linux-kernel - Re: [PATCH v13 4/6] iommu/vt-d: Add support for IOMMU default DMA mode build options

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Date:   Fri, 18 Jun 2021 09:46:24 +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 v13 4/6] iommu/vt-d: Add support for IOMMU default DMA
 mode build options

Hi John,

On 6/17/21 4:00 PM, John Garry wrote:
> On 17/06/2021 08:32, Lu Baolu wrote:
>> On 6/16/21 7:03 PM, John Garry wrote:
>>> @@ -4382,9 +4380,9 @@ int __init intel_iommu_init(void)
>>>            * is likely to be much lower than the overhead of 
>>> synchronizing
>>>            * the virtual and physical IOMMU page-tables.
>>>            */
>>> -        if (!intel_iommu_strict && cap_caching_mode(iommu->cap)) {
>>> -            pr_warn("IOMMU batching is disabled due to 
>>> virtualization");
>>> -            intel_iommu_strict = 1;
>>> +        if (cap_caching_mode(iommu->cap)) {
>>> +            pr_warn("IOMMU batching disallowed due to 
>>> virtualization\n");
>>> +            iommu_set_dma_strict(true);
>>
>> With this change, VM guest will always show this warning.
> 
> Would they have got it before also normally?
> 
> I mean, default is intel_iommu_strict=0, so if 
> cap_caching_mode(iommu->cap) is true and intel_iommu_strict not set to 1 
> elsewhere previously, then we would get this print.

Yes. You are right.

> 
>> How about
>> removing this message? Users could get the same information through the
>> kernel message added by "[PATCH v13 2/6] iommu: Print strict or lazy
>> mode at init time".
> 
> I think that the print from 2/6 should occur before this print.
> 
> Regardless I would think that you would still like to be notified of 
> this change in policy, right?
> 
> However I now realize that the print is in a loop per iommu, so we would 
> get it per iommu:
> 
> for_each_active_iommu(iommu, drhd) {
>      /*
>       * The flush queue implementation does not perform
>       * page-selective invalidations that are required for efficient
>       * TLB flushes in virtual environments.  The benefit of batching
>       * is likely to be much lower than the overhead of synchronizing
>       * the virtual and physical IOMMU page-tables.
>       */
>      if (!intel_iommu_strict && cap_caching_mode(iommu->cap)) {
>          pr_warn("IOMMU batching is disabled due to virtualization");
>          intel_iommu_strict = 1;
>      }
>      ...
> }
> 
> I need to change that. How about this:
> 
> bool print_warning = false;
> 
> for_each_active_iommu(iommu, drhd) {
>      /*
>       * The flush queue implementation does not perform
>       * page-selective invalidations that are required for efficient
>       * TLB flushes in virtual environments.  The benefit of batching
>       * is likely to be much lower than the overhead of synchronizing
>       * the virtual and physical IOMMU page-tables.
>       */
>      if (!print_warning && cap_caching_mode(iommu->cap)) {
>          pr_warn("IOMMU batching disallowed due to virtualization\n");
>          iommu_set_dma_strict(true);
>          print_warning = true;
>      }
>      ...
> }
> 
> or use pr_warn_once().

 From my p.o.v, pr_xxxx_once() is better.

How about using a pr_info_once()? I don't think it's a warning, it's
just a policy choice in VM environment.

Best regards,
baolu