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X-Proofpoint-GUID: -uM2HMqXxbeWwxygGCbuGCM0dy90q2p8 X-Authority-Analysis: v=2.4 cv=OLQqHCaB c=1 sm=1 tr=0 ts=697964fe b=1 cx=c_pps a=zPCbziy225d3KhSqZt3L1A==:117 a=zPCbziy225d3KhSqZt3L1A==:17 a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=xqWC_Br6kY4A:10 a=IkcTkHD0fZMA:10 a=vUbySO9Y5rIA:10 a=GoEa3M9JfhUA:10 a=VkNPw1HP01LnGYTKEx00:22 a=VwQbUJbxAAAA:8 a=zd2uoN0lAAAA:8 a=uQTHwiPSqmYELLZNr8EA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 cc=ntf awl=host:12103 X-Proofpoint-ORIG-GUID: -uM2HMqXxbeWwxygGCbuGCM0dy90q2p8 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMTI4MDAwOSBTYWx0ZWRfX0kt8/JwkegUJ qdP37mTbdtJNTnpvBJL8b1Twu6rJDGa2/70KpjsUBjTjn44HJSQdpG9eBcaVOFiYAyvdw3NeWKU RPu413x5AhOiSRfOhdLxYBnF3E9/wSHF/gL1YpwjlKRdyVQk9DzPW5iM8tLI8D3vDJAbLuRmMb4 g9Az2ivqM27smDvjYhdRT5fJboG65ipVO/xYz5ZUJJ0Ub3+eQvUZ0vpkPnYrQeQmMaLygPZwLCY T/Su9g/I92rBjSBGGIW09TA9JEm8yLmsWhRZ+4gEf530uPZWkbCb2ev7eD1NO9N5cu5ERb74Ptw 5aEp2mc33kt53W+6zs+bQxdaPIRuvBtaJc9V8rPH7hWqaalXcDQ2MLEZjWB9gv49P5kqoRwspxG 0Ne7SL3gsZgTpCii56S+/MO3nI8pCUP0pyAE5+0QfFIot21nn8ta0sB1Xz4KzBz6BIdO11LPkWy orYqXBEHNk/qi2+FvGtYz4N955VsksKL5t6eh4OU= Hi Sairaj, On 1/7/26 1:09 AM, Sairaj Kodilkar wrote: > Hello all, > > Gentle ping, > I mentioned privately that I am investigating the main limitations/alternatives that are listed below, but I don't yet have any suggestions as to the best way to move forward. So while I am still hoping from feedback from others in the list, I'll reply to this series as if it will be merged using the current approach. Thank you, Alejandro > On 11/18/2025 3:45 PM, Sairaj Kodilkar wrote: >> Resending this series with KVM and IOMMU maintainers in CC. >> >> AMD IOMMU can route upto 2048 MSI vectors through a single >> Interrupt Remapping Table (IRT) entry. This series brings the same >> capability to the emulated AMD IOMMU in QEMU. >> >> Highlights >> ---------- >> * Sets bits [9:8] in Extended-Feature-Register-2 to advertise 2K MSI >>    support to the guest. >> * Uses bits [10:0] of the MSI data to select the IRTE when the guest >>    programs MSIs in logical-destination mode. >> * Introduces a new IOMMU device property: >>          -device amd-iommu,...,numint2k=on >> >>    The feature is **opt-in**; guests keep the 512-MSI behaviour unless >>    `numint2k=on` is supplied. >> >> Passthrough devices >> ------------------- >> When a PCI function is passed through via iommufd the code checks the >> host’s vendor capabilities.  If the host IOMMU has not enabled >> 2K-MSI support (bits [44:43] set in the control register) the guest >> feature is disabled even if `numint2k=on` was requested. >> >> The detection logic relies on the iommufd interface; with the legacy >> VFIO container the guest always falls back to 512 MSIs. >> >> Example >> ------- >> qemu-system-x86_64 \ >> -enable-kvm -m 10G -smp cpus=8 \ >> -kernel /boot/vmlinuz \ >> -initrd /boot/initrd.img \ >> -append "console=ttyS0 earlyprintk=serial root=" >> -device amd-iommu,dma-remap=on,numint2k=on \ >> -object iommufd,id=iommufd0 \ >> -device vfio-pci,host=,iommufd=iommufd0 \ >> -global kvm-pit.lost_tick_policy=discard \ >> -cpu host \ >> -machine q35,kernel_irqchip=split \ >> -nographic \ >> -smbios type=0,version=2.8 \ >> -blockdev node- >> name=drive0,driver=qcow2,file.driver=file,file.filename= \ >> -device virtio-blk-pci,drive=drive0 >> >> Limitations >> ----------- >> This approach works well for features queried after IOMMUFD >> initialization but cannot handle features needed during early QEMU >> setup, before IOMMUFD is available. >> >> A key example is EFR2[HTRangeIgnore]. When this bit is set, the physical >> IOMMU treats HyperTransport (HT) address ranges as regular memory >> accesses rather than reserved regions. This has important implications >> for memory layout: >> >> * Without HTRangeIgnore: QEMU must relocate RAM above 4G to above 1T on >>    AMD platforms to avoid HT conflicts >> * With HTRangeIgnore: QEMU can safely place RAM immediately above 4G, >>    improving memory utilization >> >> Since RAM layout must be determined before IOMMUFD initialization, QEMU >> cannot use hwinfo to query EFR2[HTRangeIgnore] feature bit. >> >> Another limitation with using the control register is that, if BIOS enables >> particular feature (e.g. ControlRegister[GCR3TRPMode) without kernel support >> QEMU incorrectly assumes that host kernel supports that feature potentially >> causing guest failure. >> >> Alternative considered >> ---------------------- >> We also explored alternate approach which uses KVM capability >> "KVM_CAP_AMD_NUM_INT_2K_SUP", which user can query to know if host >> kernel supports 2K MSIs. Similarly, this enables qemu to detect the >> presence of EFR2[HTRangeIgnore] during RAM initialization. >> >> Although current implementation allows 2K MSI support only with >> iommufd, it keeps the logic inside the vfio/iommufd and avoids >> modifying KVM ABI. I am happy to discuss advantages and drawbacks of >> both approaches. >> >> ------------------------------------------------------------------------ >> >> The patches are based on top of bc831f37398b (qemu master). Additionally >> it requires linux kernel with patches[1] which expose control register >> via IOMMU_GET_HW_INFO ioctl. >> >> [1] https://lore.kernel.org/linux-iommu/20251029095846.4486-1- >> sarunkod@amd.com/ >> >> ------------------------------------------------------------------------ >> >> Sairaj Kodilkar (3): >>    vfio/iommufd: Add amd specific hardware info struct to vendor >>      capability >>    amd_iommu: Add support for extended feature register 2 >>    amd_iommu: Add support for upto 2048 interrupts per IRT >> >> Suravee Suthikulpanit (2): >>    [DO NOT MERGE] linux-headers: Introduce struct iommu_hw_info_amd >>    amd-iommu: Add support for set/unset IOMMU for VFIO PCI devices >> >>   hw/i386/acpi-build.c               |   4 +- >>   hw/i386/amd_iommu-stub.c           |   5 + >>   hw/i386/amd_iommu.c                | 163 +++++++++++++++++++++++++++-- >>   hw/i386/amd_iommu.h                |  24 +++++ >>   include/system/host_iommu_device.h |   1 + >>   linux-headers/linux/iommufd.h      |  20 ++++ >>   6 files changed, 207 insertions(+), 10 deletions(-) >> >