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Date: Fri, 20 Jan 2023 11:33:09 +0000
From: John Garry <john.g.garry@...cle.com>
To: Robin Murphy <robin.murphy@....com>, joro@...tes.org
Cc: will@...nel.org, iommu@...ts.linux.dev,
linux-kernel@...r.kernel.org,
Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v3] iommu: Optimise PCI SAC address trick
On 18/01/2023 17:26, Robin Murphy wrote:
> Per the reasoning in commit 4bf7fda4dce2 ("iommu/dma: Add config for
> PCI SAC address trick") and its subsequent revert, this mechanism no
> longer serves its original purpose, but now only works around broken
> hardware/drivers in a way that is unfortunately too impactful to remove.
>
> This does not, however prevent us from solving the performance impact
> which the workaround imposes on large-scale systems that don't need it.
> That is felt once the 32-bit IOVA space fills up and we keep
> unsuccessfully trying to allocate from it. However, if we get to that
> point then in fact it's already the endgame. The nature of the allocator
> is such that the first IOVA we give to a device after the 32-bit space
> runs out will be the highest possible address for that device, ever.
> If that works, then great, we can be pretty sure it's safe to optimise
> for speed by always allocating from the full range. And if it doesn't,
> then the worst has already happened and any brokenness is now showing,
> so there's no point continuing to try to hide it.
>
> To that end, implement a flag to refine this into a per-device policy
> that can automatically get itself out of the way if and when it stops
> being useful.
>
> CC: John Garry <john.garry@...wei.com>
> CC: Linus Torvalds <torvalds@...ux-foundation.org>
> Signed-off-by: Robin Murphy <robin.murphy@....com>
> ---
>
> v3: Expand the flag name, add a print with inline commentary for good
> measure, and refactor the code flow even more (too many ifs and
> indents...) such that I didn't presume to carry forward John's R-b.
I like the new changes, so feel free to add:
Reviewed-by: John Garry <john.g.garry@...cle.com>
>
> drivers/iommu/dma-iommu.c | 26 ++++++++++++++++++++------
> drivers/iommu/dma-iommu.h | 8 ++++++++
> drivers/iommu/iommu.c | 3 +++
> include/linux/iommu.h | 2 ++
> 4 files changed, 33 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index f798c44e0903..1dccee3628cf 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -627,7 +627,7 @@ static dma_addr_t iommu_dma_alloc_iova(struct iommu_domain *domain,
> {
> struct iommu_dma_cookie *cookie = domain->iova_cookie;
> struct iova_domain *iovad = &cookie->iovad;
> - unsigned long shift, iova_len, iova = 0;
> + unsigned long shift, iova_len, iova;
>
> if (cookie->type == IOMMU_DMA_MSI_COOKIE) {
> cookie->msi_iova += size;
> @@ -642,15 +642,29 @@ static dma_addr_t iommu_dma_alloc_iova(struct iommu_domain *domain,
> if (domain->geometry.force_aperture)
> dma_limit = min(dma_limit, (u64)domain->geometry.aperture_end);
>
> - /* Try to get PCI devices a SAC address */
> - if (dma_limit > DMA_BIT_MASK(32) && !iommu_dma_forcedac && dev_is_pci(dev))
> + /*
> + * Try to use all the 32-bit PCI addresses first. The original SAC vs.
> + * DAC reasoning loses relevance with PCIe, but enough hardware and
> + * firmware bugs are still lurking out there that it's safest not to
> + * venture into the 64-bit space until necessary.
> + *
> + * If your device goes wrong after seeing the notice then likely either
> + * its driver is not setting DMA masks accurately, the hardware has
> + * some inherent bug in handling >32-bit addresses, or not all the
> + * expected address bits are wired up between the device and the IOMMU.
> + */
> + if (dma_limit > DMA_BIT_MASK(32) && dev->iommu->pci_32bit_workaround) {
> iova = alloc_iova_fast(iovad, iova_len,
> DMA_BIT_MASK(32) >> shift, false);
> + if (iova)
> + goto done;
>
> - if (!iova)
> - iova = alloc_iova_fast(iovad, iova_len, dma_limit >> shift,
> - true);
> + dev->iommu->pci_32bit_workaround = false;
> + dev_notice(dev, "Using %d-bit DMA addresses\n", bits_per(dma_limit));
> + }
>
> + iova = alloc_iova_fast(iovad, iova_len, dma_limit >> shift, true);
> +done:
> return (dma_addr_t)iova << shift;
> }
>
> diff --git a/drivers/iommu/dma-iommu.h b/drivers/iommu/dma-iommu.h
> index 942790009292..c829f1f82a99 100644
> --- a/drivers/iommu/dma-iommu.h
> +++ b/drivers/iommu/dma-iommu.h
> @@ -17,6 +17,10 @@ int iommu_dma_init_fq(struct iommu_domain *domain);
> void iommu_dma_get_resv_regions(struct device *dev, struct list_head *list);
>
> extern bool iommu_dma_forcedac;
> +static inline void iommu_dma_set_pci_32bit_workaround(struct device *dev)
> +{
> + dev->iommu->pci_32bit_workaround = !iommu_dma_forcedac;
> +}
>
> #else /* CONFIG_IOMMU_DMA */
>
> @@ -38,5 +42,9 @@ static inline void iommu_dma_get_resv_regions(struct device *dev, struct list_he
> {
> }
>
> +static inline void iommu_dma_set_pci_32bit_workaround(struct device *dev)
> +{
> +}
> +
> #endif /* CONFIG_IOMMU_DMA */
> #endif /* __DMA_IOMMU_H */
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index de91dd88705b..b189ed345057 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -353,6 +353,9 @@ static int __iommu_probe_device(struct device *dev, struct list_head *group_list
> mutex_unlock(&iommu_probe_device_lock);
> iommu_device_link(iommu_dev, dev);
>
> + if (dev_is_pci(dev))
> + iommu_dma_set_pci_32bit_workaround(dev);
> +
> return 0;
>
> out_release:
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index 46e1347bfa22..d37bf28faf82 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -401,6 +401,7 @@ struct iommu_fault_param {
> * @iommu_dev: IOMMU device this device is linked to
> * @priv: IOMMU Driver private data
> * @max_pasids: number of PASIDs this device can consume
> + * @pci_32bit_workaround: Limit DMA allocations to 32-bit IOVAs
> *
> * TODO: migrate other per device data pointers under iommu_dev_data, e.g.
> * struct iommu_group *iommu_group;
> @@ -413,6 +414,7 @@ struct dev_iommu {
> struct iommu_device *iommu_dev;
> void *priv;
> u32 max_pasids;
> + bool pci_32bit_workaround;
> };
>
> int iommu_device_register(struct iommu_device *iommu,
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