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 08:31:05 +0100
From:   John Garry <john.garry@...wei.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>, <joro@...tes.org>,
        <will@...nel.org>, <dwmw2@...radead.org>, <robin.murphy@....com>,
        <corbet@....net>
CC:     <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

On 18/06/2021 02:46, Lu Baolu wrote:

Hi baolu,

>> 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.

ok, I can go with that, which Robin mostly agrees with.

Thanks,
John