Re: [RFC PATCH] iommu/arm-smmu: Add module parameter to set msi iova address

From: Robin Murphy
Date: Wed May 27 2020 - 13:30:22 EST

On 2020-05-27 17:03, Srinath Mannam wrote:
This patch gives the provision to change default value of MSI IOVA base
to platform's suitable IOVA using module parameter. The present
hardcoded MSI IOVA base may not be the accessible IOVA ranges of platform.

That in itself doesn't seem entirely unreasonable; IIRC the current address is just an arbitrary choice to fit nicely into Qemu's memory map, and there was always the possibility that it wouldn't suit everything.

Since commit aadad097cd46 ("iommu/dma: Reserve IOVA for PCIe inaccessible
DMA address"), inaccessible IOVA address ranges parsed from dma-ranges
property are reserved.

That, however, doesn't seem to fit here; iommu-dma maps MSI doorbells dynamically, so they aren't affected by reserved regions any more than regular DMA pages are. In fact, it explicitly ignores the software MSI region, since as the comment says, it *is* the software that manages those.

The MSI_IOVA_BASE region exists for VFIO, precisely because in that case the kernel *doesn't* control the address space, but still needs some way to steal a bit of it for MSIs that the guest doesn't necessarily know about, and give userspace a fighting chance of knowing what it's taken. I think at the time we discussed the idea of adding something to the VFIO uapi such that userspace could move this around if it wanted or needed to, but decided we could live without that initially. Perhaps now the time has come?


If any platform has the limitaion to access default MSI IOVA, then it can
be changed using "arm-smmu.msi_iova_base=0xa0000000" command line argument.

Signed-off-by: Srinath Mannam <srinath.mannam@xxxxxxxxxxxx>
drivers/iommu/arm-smmu.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/arm-smmu.c b/drivers/iommu/arm-smmu.c
index 4f1a350..5e59c9d 100644
--- a/drivers/iommu/arm-smmu.c
+++ b/drivers/iommu/arm-smmu.c
@@ -72,6 +72,9 @@ static bool disable_bypass =
module_param(disable_bypass, bool, S_IRUGO);
"Disable bypass streams such that incoming transactions from devices that are not attached to an iommu domain will report an abort back to the device and will not be allowed to pass through the SMMU.");
+static unsigned long msi_iova_base = MSI_IOVA_BASE;
+module_param(msi_iova_base, ulong, S_IRUGO);
+MODULE_PARM_DESC(msi_iova_base, "msi iova base address.");
struct arm_smmu_s2cr {
struct iommu_group *group;
@@ -1566,7 +1569,7 @@ static void arm_smmu_get_resv_regions(struct device *dev,
struct iommu_resv_region *region;
- region = iommu_alloc_resv_region(MSI_IOVA_BASE, MSI_IOVA_LENGTH,
+ region = iommu_alloc_resv_region(msi_iova_base, MSI_IOVA_LENGTH,
if (!region)