RISC-V IOMMU: fault-record PV (PID Valid) bit is always set to 1
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## Host environment
- Operating system:
Linux x86_64
- OS/kernel version:
Linux centos7 6.6.87.2-microsoft-standard-WSL2
- Architecture:
x86
- QEMU flavor:
qemu-system-riscv64
- QEMU version:
v11.0.0-1713-gde5d8bfd61\`, reported version 11.0.50
- QEMU command line:
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<p data-sourcepos="39:1-40:62">If you are using libvirt, virsh, or vmm, you can likely find the QEMU
command line arguments in /var/log/libvirt/qemu/$GUEST.log.--></p>
<pre data-sourcepos="42:3-51:5"><code>qemu-system-riscv64 \
-M virt,iommu-sys=on,aia=aplic-imsic \
-cpu rv64,smstateen=true \
-m 8G \
-nographic \
-device edu,dma_mask=0xFFFFFFFFFFFFFFFF \
-bios bin/iommu_fault_pv_always_set.elf
<p data-sourcepos="51:1-51:13"></code></pre></p>
<h2 id="user-content-emulatedvirtualized-environment" data-sourcepos="53:1-53:35">Emulated/Virtualized environment<a href="#emulatedvirtualized-environment" aria-label="Link to heading 'Emulated/Virtualized environment'" data-heading-content="Emulated/Virtualized environment" class="anchor"></a></h2>
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<p data-sourcepos="55:3-55:19">Operating system:</p>
<!--Windows 10 21H1, Fedora 37, etc.-->
- OS/kernel version:
<!--For POSIX guests, use `uname -a`.-->
- Architecture:
<!--x86, ARM, s390x, etc.-->
## Description of problem
The fault-record header field PV indicates that the PID field is valid, i.e. that the faulting transaction carried a valid process_id. The model sets PV to 1 unconditionally for every fault record, even when the transaction has no valid process_id. The result is fault records with PV = 1 and PID = 0 for transactions that have no process context.
## Steps to reproduce
1. [iommu_fault_pv_always_set.c](/uploads/068a503be69f8ad2daeabf0ef7570202/iommu_fault_pv_always_set.c)
2. Built with: `riscv64-unknown-elf-gcc -march=rv64gc -mabi=lp64 -mcmodel=medany -nostartfiles -nostdlib`
3. [iommu_fault_pv_always_set.elf](/uploads/3e2697368ac355c812602ecfcc86c600/iommu_fault_pv_always_set.elf)
```
qemu-system-riscv64 \
-M virt,iommu-sys=on,aia=aplic-imsic \
-cpu rv64,smstateen=true \
-m 8G \
-nographic \
-device edu,dma_mask=0xFFFFFFFFFFFFFFFF \
-bios bin/iommu_fault_pv_always_set.elf
```
## Additional information
The guest program (`tests/iommu_fault_pv_always_set.c`):
1. Registers a single-stage DC (iohgatp = Bare, iosatp = Sv39, PDTV = 0). The EDU device issues DMA without a PASID, so the transaction has no valid process_id.
2. Deliberately leaves IOVA 0x100000 unmapped and issues an EDU DMA read from it, which takes a first-stage page fault.
3. Reads the resulting fault record and inspects the PV and PID fields.
## Expected behaviour (per spec)
The transaction has no valid process_id, so the fault record must have PV = 0 (the PID field is then not meaningful).
Spec references:
- RISC-V IOMMU Architecture Specification, Section 4.2, fault record: "DID holds the device_id of the transaction. If PV is 0, then PID and PRIV are 0. If PV is 1, the PID holds a process_id of the transaction and if the privilege of the transaction was Supervisor then the PRIV bit is 1 else it's 0. The DID, PV, PID, and PRIV fields are 0 if TTYP is 0."
## Observed behaviour
```
Fault record:
CAUSE = 0x0000000D
PV = 0x00000001
PID = 0x00000000
DID = 0x00000008
BUG: fault-record PV = 1 for a transaction with no PASID (PID = 0x00000000).
PV is set unconditionally instead of reflecting process_id validity.
```
PV = 1 with PID = 0 for a transaction that has no process context.
## Root cause
In `hw/riscv/riscv-iommu.c`, `riscv_iommu_report_fault()` (master commit de5d8bfd61, lines 583-587):
```c
ev.hdr = set_field(ev.hdr, RISCV_IOMMU_FQ_HDR_DID, ctx->devid);
ev.hdr = set_field(ev.hdr, RISCV_IOMMU_FQ_HDR_PV, true); /* line 583: always true */
if (pv) {
ev.hdr = set_field(ev.hdr, RISCV_IOMMU_FQ_HDR_PID, ctx->process_id);
}
```
The function receives a `pv` argument indicating whether the PID is valid, and the callers compute it correctly (e.g. line 740 passes `!!ctx->process_id`). The `pv` argument is then used only to decide whether to write the PID field; the PV header bit itself is set to `true` unconditionally at line 583.
## Notes
- Effect: any consumer of the fault queue reads PV = 1 and treats PID = 0 as a valid process_id for faults that had no process context.
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issue
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