linux-kernel - Re: [PATCH] iommu/vt-d: do not use flush-queue when caching-mode is on

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Date:   Wed, 27 Jan 2021 08:29:37 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Nadav Amit <nadav.amit@...il.com>, iommu@...ts.linux-foundation.org
Cc:     baolu.lu@...ux.intel.com, linux-kernel@...r.kernel.org,
        Nadav Amit <namit@...are.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] iommu/vt-d: do not use flush-queue when caching-mode is
 on

On 1/27/21 8:26 AM, Lu Baolu wrote:
>> +{
>> +    struct dmar_domain *dmar_domain = to_dmar_domain(domain);
>> +    struct intel_iommu *iommu = domain_get_iommu(dmar_domain);
>> +
>> +    if (intel_iommu_strict)
>> +        return 0;
>> +
>> +    /*
>> +     * 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 (iommu && cap_caching_mode(iommu->cap)) {
>> +        pr_warn_once("IOMMU batching is partially disabled due to 
>> virtualization");
>> +        return 0;
>> +    }
> 
> domain_get_iommu() only returns the first iommu, and could return NULL
> when this is called before domain attaching to any device. A better
> choice could be check caching mode globally and return false if caching
> mode is supported on any iommu.
> 
>         struct dmar_drhd_unit *drhd;
>         struct intel_iommu *iommu;
> 
>         rcu_read_lock();
>         for_each_active_iommu(iommu, drhd) {
>                  if (cap_caching_mode(iommu->cap))
>                          return false;

We should unlock rcu before return here. Sorry!

>          }
>          rcu_read_unlock();

Best regards,
baolu