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Subject: Re: [PATCH v13 4/6] iommu/vt-d: Add support for IOMMU default DMA
 mode build options
To:     Lu Baolu <baolu.lu@linux.intel.com>, <joro@8bytes.org>,
        <will@kernel.org>, <dwmw2@infradead.org>, <robin.murphy@arm.com>,
        <corbet@lwn.net>
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        <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>
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 <46dbce5c-1c2b-60d4-df56-d2b95a959425@linux.intel.com>
From:   John Garry <john.garry@huawei.com>
Message-ID: <f3fe6c4b-f360-ab7b-7ad2-ced63269499d@huawei.com>
Date:   Thu, 17 Jun 2021 09:00:37 +0100
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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.

> 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().

Thanks,
John