From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Garry Subject: Re: [PATCH 1/3] iommu: Add fast hook for getting DMA domains Date: Fri, 17 Aug 2018 10:36:05 +0100 Message-ID: References: <4d50587e62c16f7b5cfc45dee808d774655155cf.1534250425.git.robin.murphy@arm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4d50587e62c16f7b5cfc45dee808d774655155cf.1534250425.git.robin.murphy@arm.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=m.gmane.org@lists.infradead.org To: Robin Murphy , joro@8bytes.org, will.deacon@arm.com, catalin.marinas@arm.com Cc: thunder.leizhen@huawei.com, linuxarm@huawei.com, iommu@lists.linux-foundation.org, liudongdong3@huawei.com, guohanjun@huawei.com, linux-arm-kernel@lists.infradead.org List-Id: iommu@lists.linux-foundation.org On 14/08/2018 14:04, Robin Murphy wrote: > While iommu_get_domain_for_dev() is the robust way for arbitrary IOMMU > API callers to retrieve the domain pointer, for DMA ops domains it > doesn't scale well for large systems and multi-queue devices, since the > momentary refcount adjustment will lead to exclusive cacheline contention > when multiple CPUs are operating in parallel on different mappings for > the same device. > > In the case of DMA ops domains, however, this refcounting is actually > unnecessary, since they already imply that the group exists and is > managed by platform code and IOMMU internals (by virtue of > iommu_group_get_for_dev()) such that a reference will already be held > for the lifetime of the device. Thus we can avoid the bottleneck by > providing a fast lookup specifically for the DMA code to retrieve the > default domain it already knows it has set up - a simple read-only > dereference plays much nicer with cache-coherency protocols. > > Signed-off-by: Robin Murphy > --- > drivers/iommu/iommu.c | 9 +++++++++ > include/linux/iommu.h | 1 + > 2 files changed, 10 insertions(+) > > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c > index 63b37563db7e..63c586875df5 100644 > --- a/drivers/iommu/iommu.c > +++ b/drivers/iommu/iommu.c > @@ -1379,6 +1379,15 @@ struct iommu_domain *iommu_get_domain_for_dev(struct device *dev) > } > EXPORT_SYMBOL_GPL(iommu_get_domain_for_dev); > > +/* > + * For IOMMU_DOMAIN_DMA implementations which already provide their own > + * guarantees that the group and its default domain are valid and correct. > + */ > +struct iommu_domain *iommu_get_dma_domain(struct device *dev) > +{ > + return dev->iommu_group->default_domain; > +} > + > /* > * IOMMU groups are really the natrual working unit of the IOMMU, but > * the IOMMU API works on domains and devices. Bridge that gap by > diff --git a/include/linux/iommu.h b/include/linux/iommu.h > index 19938ee6eb31..16f2172698e5 100644 > --- a/include/linux/iommu.h > +++ b/include/linux/iommu.h > @@ -297,6 +297,7 @@ extern int iommu_attach_device(struct iommu_domain *domain, > extern void iommu_detach_device(struct iommu_domain *domain, > struct device *dev); > extern struct iommu_domain *iommu_get_domain_for_dev(struct device *dev); > +extern struct iommu_domain *iommu_get_dma_domain(struct device *dev); Hi Robin, I was wondering whether it's standard to provide a stubbed version of this function for !CONFIG_IOMMU_API? Cheers, John > extern int iommu_map(struct iommu_domain *domain, unsigned long iova, > phys_addr_t paddr, size_t size, int prot); > extern size_t iommu_unmap(struct iommu_domain *domain, unsigned long iova, >