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Message-ID: <20250427081312.GE5848@unreal>
Date: Sun, 27 Apr 2025 11:13:12 +0300
From: Leon Romanovsky <leon@...nel.org>
To: Luis Chamberlain <mcgrof@...nel.org>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
Keith Busch <kbusch@...nel.org>, Jake Edge <jake@....net>,
Jonathan Corbet <corbet@....net>, Jason Gunthorpe <jgg@...pe.ca>,
Zhu Yanjun <zyjzyj2000@...il.com>,
Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
Will Deacon <will@...nel.org>, Sagi Grimberg <sagi@...mberg.me>,
Bjorn Helgaas <bhelgaas@...gle.com>,
Logan Gunthorpe <logang@...tatee.com>,
Yishai Hadas <yishaih@...dia.com>,
Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>,
Kevin Tian <kevin.tian@...el.com>,
Alex Williamson <alex.williamson@...hat.com>,
Jérôme Glisse <jglisse@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-block@...r.kernel.org, linux-rdma@...r.kernel.org,
iommu@...ts.linux.dev, linux-nvme@...ts.infradead.org,
linux-pci@...r.kernel.org, kvm@...r.kernel.org, linux-mm@...ck.org,
Niklas Schnelle <schnelle@...ux.ibm.com>,
Chuck Lever <chuck.lever@...cle.com>,
Matthew Wilcox <willy@...radead.org>,
Dan Williams <dan.j.williams@...el.com>,
Kanchan Joshi <joshi.k@...sung.com>,
Chaitanya Kulkarni <kch@...dia.com>
Subject: Re: [PATCH v9 07/24] dma-mapping: Implement link/unlink ranges API
On Sat, Apr 26, 2025 at 03:46:30PM -0700, Luis Chamberlain wrote:
> On Wed, Apr 23, 2025 at 11:12:58AM +0300, Leon Romanovsky wrote:
> > From: Leon Romanovsky <leonro@...dia.com>
> >
> > Introduce new DMA APIs to perform DMA linkage of buffers
> > in layers higher than DMA.
> >
> > In proposed API, the callers will perform the following steps.
> > In map path:
> > if (dma_can_use_iova(...))
> > dma_iova_alloc()
> > for (page in range)
> > dma_iova_link_next(...)
> > dma_iova_sync(...)
> > else
> > /* Fallback to legacy map pages */
> > for (all pages)
> > dma_map_page(...)
> >
> > In unmap path:
> > if (dma_can_use_iova(...))
> > dma_iova_destroy()
> > else
> > for (all pages)
> > dma_unmap_page(...)
> >
> > Reviewed-by: Christoph Hellwig <hch@....de>
> > Tested-by: Jens Axboe <axboe@...nel.dk>
> > Signed-off-by: Leon Romanovsky <leonro@...dia.com>
> > ---
> > drivers/iommu/dma-iommu.c | 261 ++++++++++++++++++++++++++++++++++++
> > include/linux/dma-mapping.h | 32 +++++
> > 2 files changed, 293 insertions(+)
> >
> > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> > index d2c298083e0a..2e014db5a244 100644
> > --- a/drivers/iommu/dma-iommu.c
> > +++ b/drivers/iommu/dma-iommu.c
> > @@ -1818,6 +1818,267 @@ void dma_iova_free(struct device *dev, struct dma_iova_state *state)
> > }
> > EXPORT_SYMBOL_GPL(dma_iova_free);
> >
> > +static int __dma_iova_link(struct device *dev, dma_addr_t addr,
> > + phys_addr_t phys, size_t size, enum dma_data_direction dir,
> > + unsigned long attrs)
> > +{
> > + bool coherent = dev_is_dma_coherent(dev);
> > +
> > + if (!coherent && !(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> > + arch_sync_dma_for_device(phys, size, dir);
>
> So arch_sync_dma_for_device() is a no-op on some architectures, notably x86.
> So since you're doing this work and given the above pattern is common on
> the non iova case, we could save ourselves 2 branches checks on x86 on
> __dma_iova_link() and also generalize savings for the non-iova case as
> well. For the non-iova case we have two use cases, one with the attrs on
> initial mapping, and one without on subsequent sync ops. For the iova
> case the attr is always consistently used.
I want to believe that compiler will discards these "if (!coherent &&
!(attrs & DMA_ATTR_SKIP_CPU_SYNC)))" branch if case is empty.
>
> So we could just have something like this:
>
> #ifdef CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE
> static inline void arch_sync_dma_device(struct device *dev,
> phys_addr_t paddr, size_t size,
> enum dma_data_direction dir)
> {
> if (!dev_is_dma_coherent(dev))
> arch_sync_dma_for_device(paddr, size, dir);
> }
>
> static inline void arch_sync_dma_device_attrs(struct device *dev,
> phys_addr_t paddr, size_t size,
> enum dma_data_direction dir,
> unsigned long attrs)
> {
> if (!dev_is_dma_coherent(dev) && !(attrs & DMA_ATTR_SKIP_CPU_SYNC))
> arch_sync_dma_for_device(paddr, size, dir);
> }
> #else
> static inline void arch_sync_dma_device(struct device *dev,
> phys_addr_t paddr, size_t size,
> enum dma_data_direction dir)
> {
> }
>
> static inline void arch_sync_dma_device_attrs(struct device *dev,
> phys_addr_t paddr, size_t size,
> enum dma_data_direction dir,
> unsigned long attrs)
> {
> }
> #endif
The problem is that dev_is_dma_coherent() and DMA_ATTR_SKIP_CPU_SYNC
checks are scattered over all dma-iommu.c file with different
combinations. While we can do new static functions for small number of
use cases, it will be half-solution.
>
> > +/**
> > + * dma_iova_link - Link a range of IOVA space
> > + * @dev: DMA device
> > + * @state: IOVA state
> > + * @phys: physical address to link
> > + * @offset: offset into the IOVA state to map into
> > + * @size: size of the buffer
> > + * @dir: DMA direction
> > + * @attrs: attributes of mapping properties
> > + *
> > + * Link a range of IOVA space for the given IOVA state without IOTLB sync.
> > + * This function is used to link multiple physical addresses in contiguous
> > + * IOVA space without performing costly IOTLB sync.
> > + *
> > + * The caller is responsible to call to dma_iova_sync() to sync IOTLB at
> > + * the end of linkage.
> > + */
> > +int dma_iova_link(struct device *dev, struct dma_iova_state *state,
> > + phys_addr_t phys, size_t offset, size_t size,
> > + enum dma_data_direction dir, unsigned long attrs)
> > +{
> > + struct iommu_domain *domain = iommu_get_dma_domain(dev);
> > + struct iommu_dma_cookie *cookie = domain->iova_cookie;
> > + struct iova_domain *iovad = &cookie->iovad;
> > + size_t iova_start_pad = iova_offset(iovad, phys);
> > +
> > + if (WARN_ON_ONCE(iova_start_pad && offset > 0))
> > + return -EIO;
> > +
> > + if (dev_use_swiotlb(dev, size, dir) && iova_offset(iovad, phys | size))
>
> There is already a similar check for the non-iova case for this on
> iommu_dma_map_page() and a nice comment about what why this checked,
> this seems to be just screaming for a helper:
>
> /*
> * Checks if a physical buffer has unaligned boundaries with respect to
> * the IOMMU granule. Returns non-zero if either the start or end
> * address is not aligned to the granule boundary.
> */
> static inline size_t iova_unaligned(struct iova_domain *iovad,
> phys_addr_t phys,
> size_t size)
> {
> return iova_offset(iovad, phys | size);
> }
I added this function, thanks.
> Other than that, looks good.
>
> Reviewed-by: Luis Chamberlain <mcgrof@...nel.org>
>
> Luis
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