Messages in this thread | | | Date | Wed, 10 Apr 2024 14:30:20 +0800 | Subject | Re: [PATCH v2 1/1] iommu/vt-d: Remove caching mode check before device TLB flush | From | Yi Liu <> |
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On 2024/4/10 13:58, Lu Baolu wrote: > The Caching Mode (CM) of the Intel IOMMU indicates if the hardware > implementation caches not-present or erroneous translation-structure > entries except the first-stage translation. The caching mode is > irrelevant to the device TLB , therefore there is no need to check > it before a device TLB invalidation operation. > > iommu_flush_iotlb_psi() is called in map and unmap paths. The caching > mode check before device TLB invalidation will cause device TLB > invalidation always issued if IOMMU is not running in caching mode. > This is wrong and causes unnecessary performance overhead.
I don't think the original code is wrong. As I replied before, if CM==0, the iommu_flush_iotlb_psi() is only called in unmap path, in which the @map is false. [1] The reason to make the change is to make the logic simpler. :)
[1] https://lore.kernel.org/linux-iommu/ac7fdf0f-ecb9-415f-9c1b-600536b92ca5@intel.com/
> The removal of caching mode check in intel_flush_iotlb_all() doesn't > impact anything no matter the IOMMU is working in caching mode or not. > Commit <29b32839725f> ("iommu/vt-d: Do not use flush-queue when > caching-mode is on") has already disabled flush-queue for caching mode, > hence caching mode will never call intel_flush_iotlb_all(). > > Fixes: bf92df30df90 ("intel-iommu: Only avoid flushing device IOTLB for domain ID 0 in caching mode") > Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com> > --- > drivers/iommu/intel/iommu.c | 9 ++------- > 1 file changed, 2 insertions(+), 7 deletions(-) > > Change log: > > v2: > - Squash two patches into a single one. > - No functionality changes. > > v1: https://lore.kernel.org/linux-iommu/20240407144232.190355-1-baolu.lu@linux.intel.com/ > > diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c > index 50eb9aed47cc..681789b1258d 100644 > --- a/drivers/iommu/intel/iommu.c > +++ b/drivers/iommu/intel/iommu.c > @@ -1501,11 +1501,7 @@ static void iommu_flush_iotlb_psi(struct intel_iommu *iommu, > else > __iommu_flush_iotlb_psi(iommu, did, pfn, pages, ih); > > - /* > - * In caching mode, changes of pages from non-present to present require > - * flush. However, device IOTLB doesn't need to be flushed in this case. > - */ > - if (!cap_caching_mode(iommu->cap) || !map) > + if (!map) > iommu_flush_dev_iotlb(domain, addr, mask); > } > > @@ -1579,8 +1575,7 @@ static void intel_flush_iotlb_all(struct iommu_domain *domain) > iommu->flush.flush_iotlb(iommu, did, 0, 0, > DMA_TLB_DSI_FLUSH); > > - if (!cap_caching_mode(iommu->cap)) > - iommu_flush_dev_iotlb(dmar_domain, 0, MAX_AGAW_PFN_WIDTH); > + iommu_flush_dev_iotlb(dmar_domain, 0, MAX_AGAW_PFN_WIDTH);
why not one more step to move it out of the iommu_array loop? There is no more reason to keep it in the loop.
> } > > if (dmar_domain->nested_parent)
-- Regards, Yi Liu
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