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

[Lu Baolu]

Hi Nadav,

On 1/27/21 4:38 AM, Nadav Amit wrote:
> From: Nadav Amit <namit@xxxxxxxxxx>
>
> When an Intel IOMMU is virtualized, and a physical device is
> passed-through to the VM, changes of the virtual IOMMU need to be
> propagated to the physical IOMMU. The hypervisor therefore needs to
> monitor PTE mappings in the IOMMU page-tables. Intel specifications
> provide "caching-mode" capability that a virtual IOMMU uses to report
> that the IOMMU is virtualized and a TLB flush is needed after mapping to
> allow the hypervisor to propagate virtual IOMMU mappings to the physical
> IOMMU. To the best of my knowledge no real physical IOMMU reports
> "caching-mode" as turned on.
>
> Synchronizing the virtual and the physical TLBs is expensive if the
> hypervisor is unaware which PTEs have changed, as the hypervisor is
> required to walk all the virtualized tables and look for changes.
> Consequently, domain flushes are much more expensive than page-specific
> flushes on virtualized IOMMUs with passthrough devices. The kernel
> therefore exploited the "caching-mode" indication to avoid domain
> flushing and use page-specific flushing in virtualized environments. See
> commit 78d5f0f500e6 ("intel-iommu: Avoid global flushes with caching
> mode.")
>
> This behavior changed after commit 13cf01744608 ("iommu/vt-d: Make use
> of iova deferred flushing"). Now, when batched TLB flushing is used (the
> default), full TLB domain flushes are performed frequently, requiring
> the hypervisor to perform expensive synchronization between the virtual
> TLB and the physical one.

Good catch. Thank you!

> Getting batched TLB flushes to use in such circumstances page-specific
> invalidations again is not easy, since the TLB invalidation scheme
> assumes that "full" domain TLB flushes are performed for scalability.
>
> Disable batched TLB flushes when caching-mode is on, as the performance
> benefit from using batched TLB invalidations is likely to be much
> smaller than the overhead of the virtual-to-physical IOMMU page-tables
> synchronization.
>
> Fixes: 78d5f0f500e6 ("intel-iommu: Avoid global flushes with caching mode.")
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
> Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
> Cc: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> Cc: Joerg Roedel <joro@xxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>   drivers/iommu/intel/iommu.c | 26 +++++++++++++++++++++++++-
>   1 file changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 788119c5b021..4e08f5e17175 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -5373,6 +5373,30 @@ intel_iommu_domain_set_attr(struct iommu_domain *domain,
>   	return ret;
>   }
>   
> +static int
> +intel_iommu_domain_get_attr_use_flush_queue(struct iommu_domain *domain)

Just nit:

Can we just use this

static bool domain_use_flush_queue(struct iommu_domain *domain)

?

> +{
> +	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;
        }
        rcu_read_unlock();

> +
> +	return 1;
> +}
> +
>   static int
>   intel_iommu_domain_get_attr(struct iommu_domain *domain,
>   			    enum iommu_attr attr, void *data)
> @@ -5383,7 +5407,7 @@ intel_iommu_domain_get_attr(struct iommu_domain *domain,
>   	case IOMMU_DOMAIN_DMA:
>   		switch (attr) {
>   		case DOMAIN_ATTR_DMA_USE_FLUSH_QUEUE:
> -			*(int *)data = !intel_iommu_strict;
> +			*(int *)data = !intel_iommu_domain_get_attr_use_flush_queue(domain);
>   			return 0;
>   		default:
>   			return -ENODEV;

Best regards,
baolu