Messages in this thread | | | Date | Tue, 17 May 2022 10:38:34 +0200 | From | Christoph Hellwig <> | Subject | Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping_size() |
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On Mon, May 16, 2022 at 09:06:01PM +0800, John Garry wrote: > For streaming DMA mappings involving an IOMMU and whose IOVA len regularly > exceeds the IOVA rcache upper limit (meaning that they are not cached), > performance can be reduced. > > Add the IOMMU callback for DMA mapping API dma_max_mapping_size(), which > allows the drivers to know the mapping limit and thus limit the requested > IOVA lengths. > > This resolves the performance issue originally reported in [0] for a SCSI > HBA driver which was regularly mapping SGLs which required IOVAs in > excess of the IOVA caching limit. In this case the block layer limits the > max sectors per request - as configured in __scsi_init_queue() - which > will limit the total SGL length the driver tries to map and in turn limits > IOVA lengths requested. > > [0] https://lore.kernel.org/linux-iommu/20210129092120.1482-1-thunder.leizhen@huawei.com/ > > Signed-off-by: John Garry <john.garry@huawei.com> > --- > Sending as an RFC as iommu_dma_max_mapping_size() is a soft limit, and not > a hard limit which I expect is the semantics of dma_map_ops.max_mapping_size > > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c > index 09f6e1c0f9c0..e2d5205cde37 100644 > --- a/drivers/iommu/dma-iommu.c > +++ b/drivers/iommu/dma-iommu.c > @@ -1442,6 +1442,21 @@ static unsigned long iommu_dma_get_merge_boundary(struct device *dev) > return (1UL << __ffs(domain->pgsize_bitmap)) - 1; > } >
> + if (!domain) > + return 0; > + > + cookie = domain->iova_cookie; > + if (!cookie || cookie->type != IOMMU_DMA_IOVA_COOKIE) > + return 0;
Can these conditions even be true here?
> +static inline unsigned long iova_rcache_range(void) > +{ > + return 0; > +}
Given that IOMMU_DMA select IOMMU_IOVA there is no need for this stub.
Otherwise this looks sensible to me.
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