On Thu, Feb 04, 2021 at 06:04:13PM +0000, Nadav Amit wrote:
> Backporting requires to disable strict during initialization. Lu, can
> you ack this patch?
>
>
> -- >8 --
>
> >From d5ce982ce6f6f869c53cc0ed496a6dd4c1309657 Mon Sep 17 00:00:00 2001
> From: Nadav Amit <namit@xxxxxxxxxx>
> Date: Tue, 26 Jan 2021 12:03:11 -0800
> Subject: [PATCH] iommu/vt-d: do not use flush-queue when caching-mode is on
>
> 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 page-specific invalidations again in
> such circumstances 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: 13cf01744608 ("iommu/vt-d: Make use of iova deferred flushing")
> 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 | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 151243fa01ba..7e3db4c0324d 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -3350,6 +3350,11 @@ static int __init init_dmars(void)
>
> if (!ecap_pass_through(iommu->ecap))
> hw_pass_through = 0;
> +
> + if (!intel_iommu_strict && cap_caching_mode(iommu->cap)) {
> + pr_warn("Disable batched IOTLB flush due to virtualization");
> + intel_iommu_strict = 1;
> + }
> intel_svm_check(iommu);
> }
>
This works for 5.10, thanks! But what about 4.9, 4.14, 4.19, and 5.4?
Those also need this change, right?
thanks,
greg k-h
