On 1/29/25 3:58 PM, Jason Gunthorpe wrote: > On Wed, Jan 29, 2025 at 02:44:12PM +0100, Eric Auger wrote: >> Hi, >> >> >> On 1/11/25 4:32 AM, Nicolin Chen wrote: >>> For systems that require MSI pages to be mapped into the IOMMU translation >>> the IOMMU driver provides an IOMMU_RESV_SW_MSI range, which is the default >>> recommended IOVA window to place these mappings. However, there is nothing >>> special about this address. And to support the RMR trick in VMM for nested >> well at least it shall not overlap VMM's RAM. So it was not random either. >>> translation, the VMM needs to know what sw_msi window the kernel is using. >>> As there is no particular reason to force VMM to adopt the kernel default, >>> provide a simple IOMMU_OPTION_SW_MSI_START/SIZE ioctl that the VMM can use >>> to directly specify the sw_msi window that it wants to use, which replaces >>> and disables the default IOMMU_RESV_SW_MSI from the driver to avoid having >>> to build an API to discover the default IOMMU_RESV_SW_MSI. >> IIUC the MSI window will then be different when using legacy VFIO >> assignment and iommufd backend. > ? They use the same, iommufd can have userspace override it. Then it > will ignore the reserved region. In current arm-smmu-v3.c you have region = iommu_alloc_resv_region(MSI_IOVA_BASE, MSI_IOVA_LENGTH, prot, IOMMU_RESV_SW_MSI, GFP_KERNEL); in arm_smmu_get_resv_regions() If you overwrite the default region, don't you need to expose the user defined resv region? > >> MSI reserved regions are exposed in >> /sys/kernel/iommu_groups/<n>/reserved_regions >> 0x0000000008000000 0x00000000080fffff msi > >> Is that configurability reflected accordingly? > ? > > Nothing using iommufd should parse that sysfs file. Right but aren't you still supposed to populate the sysfs files properly. This region must be carved out from the IOVA space, right? > >> How do you make sure it does not collide with other resv regions? I >> don't see any check here. > Yes this does need to be checked, it does look missing. It still needs > to create a reserved region in the ioas when attaching to keep the > areas safe and it has to intersect with the incoming reserved > regions from the driver. > >>> + * @IOMMU_OPTION_SW_MSI_START: >>> + * Change the base address of the IOMMU mapping region for MSI doorbell(s). >>> + * It must be set this before attaching a device to an IOAS/HWPT, otherwise >>> + * this option will be not effective on that IOAS/HWPT. User can choose to >>> + * let kernel pick a base address, by simply ignoring this option or setting >>> + * a value 0 to IOMMU_OPTION_SW_MSI_SIZE. Global option, object_id must be 0 >> I think we should document it cannot be put at a random place either. > It can be put at any place a map can be placed. to me It cannot overlap with guest RAM IPA so userspace needs to be cautious about that Eric > > That also needs to be checked when creating a domain, it can't be > outside the geometry. > > Jason >
