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    SubjectRe: [PATCH] iommu/dma: Resurrect the "forcedac" option
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    Hi Robin,

    On 3/6/21 12:32 AM, Robin Murphy wrote:
    > In converting intel-iommu over to the common IOMMU DMA ops, it quietly
    > lost the functionality of its "forcedac" option. Since this is a handy
    > thing both for testing and for performance optimisation on certain
    > platforms, reimplement it under the common IOMMU parameter namespace.
    >
    > For the sake of fixing the inadvertent breakage of the Intel-specific
    > parameter, remove the dmar_forcedac remnants and hook it up as an alias
    > while documenting the transition to the new common parameter.
    >
    > Fixes: c588072bba6b ("iommu/vt-d: Convert intel iommu driver to the iommu ops")
    > Signed-off-by: Robin Murphy <robin.murphy@arm.com>
    > ---
    > Documentation/admin-guide/kernel-parameters.txt | 15 ++++++++-------
    > drivers/iommu/dma-iommu.c | 13 ++++++++++++-
    > drivers/iommu/intel/iommu.c | 5 ++---
    > include/linux/dma-iommu.h | 2 ++
    > 4 files changed, 24 insertions(+), 11 deletions(-)
    >
    > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
    > index 04545725f187..835f810f2f26 100644
    > --- a/Documentation/admin-guide/kernel-parameters.txt
    > +++ b/Documentation/admin-guide/kernel-parameters.txt
    > @@ -1869,13 +1869,6 @@
    > bypassed by not enabling DMAR with this option. In
    > this case, gfx device will use physical address for
    > DMA.
    > - forcedac [X86-64]
    > - With this option iommu will not optimize to look
    > - for io virtual address below 32-bit forcing dual
    > - address cycle on pci bus for cards supporting greater
    > - than 32-bit addressing. The default is to look
    > - for translation below 32-bit and if not available
    > - then look in the higher range.
    > strict [Default Off]
    > With this option on every unmap_single operation will
    > result in a hardware IOTLB flush operation as opposed
    > @@ -1964,6 +1957,14 @@
    > nobypass [PPC/POWERNV]
    > Disable IOMMU bypass, using IOMMU for PCI devices.
    >
    > + iommu.forcedac= [ARM64, X86] Control IOVA allocation for PCI devices.
    > + Format: { "0" | "1" }
    > + 0 - Try to allocate a 32-bit DMA address first, before
    > + falling back to the full range if needed.
    > + 1 - Allocate directly from the full usable range,
    > + forcing Dual Address Cycle for PCI cards supporting
    > + greater than 32-bit addressing.
    > +
    > iommu.strict= [ARM64] Configure TLB invalidation behaviour
    > Format: { "0" | "1" }
    > 0 - Lazy mode.
    > diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
    > index 9ab6ee22c110..260bf3de1992 100644
    > --- a/drivers/iommu/dma-iommu.c
    > +++ b/drivers/iommu/dma-iommu.c
    > @@ -52,6 +52,17 @@ struct iommu_dma_cookie {
    > };
    >
    > static DEFINE_STATIC_KEY_FALSE(iommu_deferred_attach_enabled);
    > +bool iommu_dma_forcedac __read_mostly;
    > +
    > +static int __init iommu_dma_forcedac_setup(char *str)
    > +{
    > + int ret = kstrtobool(str, &iommu_dma_forcedac);
    > +
    > + if (!ret && iommu_dma_forcedac)
    > + pr_info("Forcing DAC for PCI devices\n");
    > + return ret;
    > +}
    > +early_param("iommu.forcedac", iommu_dma_forcedac_setup);
    >
    > void iommu_dma_free_cpu_cached_iovas(unsigned int cpu,
    > struct iommu_domain *domain)
    > @@ -438,7 +449,7 @@ static dma_addr_t iommu_dma_alloc_iova(struct iommu_domain *domain,
    > 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) && dev_is_pci(dev))
    > + if (dma_limit > DMA_BIT_MASK(32) && !iommu_dma_forcedac && dev_is_pci(dev))
    > iova = alloc_iova_fast(iovad, iova_len,
    > DMA_BIT_MASK(32) >> shift, false);
    >
    > diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
    > index ee0932307d64..1c32522220bc 100644
    > --- a/drivers/iommu/intel/iommu.c
    > +++ b/drivers/iommu/intel/iommu.c
    > @@ -360,7 +360,6 @@ int intel_iommu_enabled = 0;
    > EXPORT_SYMBOL_GPL(intel_iommu_enabled);
    >
    > static int dmar_map_gfx = 1;
    > -static int dmar_forcedac;
    > static int intel_iommu_strict;
    > static int intel_iommu_superpage = 1;
    > static int iommu_identity_mapping;
    > @@ -451,8 +450,8 @@ static int __init intel_iommu_setup(char *str)
    > dmar_map_gfx = 0;
    > pr_info("Disable GFX device mapping\n");
    > } else if (!strncmp(str, "forcedac", 8)) {
    > - pr_info("Forcing DAC for PCI devices\n");
    > - dmar_forcedac = 1;
    > + pr_warn("intel_iommu=forcedac deprecated; use iommu.forcedac instead\n");
    > + iommu_dma_forcedac = true;
    > } else if (!strncmp(str, "strict", 6)) {
    > pr_info("Disable batched IOTLB flush\n");
    > intel_iommu_strict = 1;
    > diff --git a/include/linux/dma-iommu.h b/include/linux/dma-iommu.h
    > index 706b68d1359b..13d1f4c14d7b 100644
    > --- a/include/linux/dma-iommu.h
    > +++ b/include/linux/dma-iommu.h
    > @@ -40,6 +40,8 @@ void iommu_dma_get_resv_regions(struct device *dev, struct list_head *list);
    > void iommu_dma_free_cpu_cached_iovas(unsigned int cpu,
    > struct iommu_domain *domain);
    >
    > +extern bool iommu_dma_forcedac;
    > +
    > #else /* CONFIG_IOMMU_DMA */
    >
    > struct iommu_domain;
    >

    Thanks for catching this.

    Acked-by: Lu Baolu <baolu.lu@linux.intel.com>

    Best regards,
    baolu

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