Re: [PATCH v2] iommu/vt-d: do not use flush-queue when caching-mode is on

Lu Baolu
2021-01-27T11:29:04Z

On 2021/1/27 14:17, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>
>
> 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 IOMMU tables 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.
>
> 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.")

Isn't it

Fixes: 13cf01744608 ("iommu/vt-d: Make use of iova deferred flushing")

?

Best regards,
baolu

> Signed-off-by: Nadav Amit <namit@vmware.com>
> Cc: David Woodhouse <dwmw2@infradead.org>
> Cc: Lu Baolu <baolu.lu@linux.intel.com>
> Cc: Joerg Roedel <joro@8bytes.org>
> Cc: Will Deacon <will@kernel.org>
> Cc: stable@vger.kernel.org >
> ---
> v1->v2:
>
> * disable flush queue for all domains if caching-mode is on for any
> IOMMU (Lu).
> ---
> drivers/iommu/intel/iommu.c | 32 +++++++++++++++++++++++++++++++-
> 1 file changed, 31 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 788119c5b021..de3dd617cf60 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -5373,6 +5373,36 @@ intel_iommu_domain_set_attr(struct iommu_domain *domain,
> return ret;
> }
>
> +static bool domain_use_flush_queue(void)
> +{
> + struct dmar_drhd_unit *drhd;
> + struct intel_iommu *iommu;
> + bool r = true;
> +
> + if (intel_iommu_strict)
> + return false;
> +
> + /*
> + * 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.
> + */
> + rcu_read_lock();
> + for_each_active_iommu(iommu, drhd) {
> + if (!cap_caching_mode(iommu->cap))
> + continue;
> +
> + pr_warn_once("IOMMU batching is disabled due to virtualization");
> + r = false;
> + break;
> + }
> + rcu_read_unlock();
> +
> + return r;
> +}
> +
> static int
> intel_iommu_domain_get_attr(struct iommu_domain *domain,
> enum iommu_attr attr, void *data)
> @@ -5383,7 +5413,7 @@ 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 = domain_use_flush_queue();
> return 0;
> default:
> return -ENODEV;
>