| Message ID | 4d50587e62c16f7b5cfc45dee808d774655155cf.1534250425.git.robin.murphy@arm.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | iommu: Avoid DMA ops domain refcount contention | expand |
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 <robin.murphy@arm.com> > --- > 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, >
On 17/08/18 10:36, John Garry wrote: > 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 <robin.murphy@arm.com> >> --- >> 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? Nope, that's deliberate - this is one of those hooks where any legitimate caller will already be dependent on IOMMU_API itself. Robin. > > 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, >> > >
Hi Robin, On 14.08.2018 16:04, 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 <robin.murphy@arm.com> > --- > 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; > +} After some preliminary tests I'm seeing a ~10% performance improvement on one of our chips (nxp ls1046a) which is pretty nice. :-) Any chance of making the function inline? If not, shouldn't an EXPORT_SYMBOL_GPL be added? --- Thanks & Best Regards, Laurentiu
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); 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,
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 <robin.murphy@arm.com> --- drivers/iommu/iommu.c | 9 +++++++++ include/linux/iommu.h | 1 + 2 files changed, 10 insertions(+)