RISC-V IOMMU: fault-record TTYP reports a write for read transactions (UADDR_RD never used)
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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:
<!--Give the smallest, complete command line that exhibits the problem.
</li>
</ul>
<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="42:3-50:5"><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_ttyp_read.elf
</code></pre>
<h2 id="user-content-emulatedvirtualized-environment" data-sourcepos="52:1-52:35">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>
<ul data-sourcepos="54:1-62:28">
<li data-sourcepos="54:1-56:41">
<p data-sourcepos="54:3-54:19">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
When a transaction faults, the fault-record transaction type (TTYP) is chosen with a mask test that is wrong: read transactions are recorded as writes (UADDR_WR). The read transaction type (UADDR_RD) is never produced. The CAUSE field is computed separately and correctly, so a single fault record can carry CAUSE = "read page fault" together with TTYP = "untranslated write", which contradict each other.
## Steps to reproduce
1. [iommu_fault_ttyp_read.c](/uploads/d33eec74f71610b3bffeafc3aad1567b/iommu_fault_ttyp_read.c)
2. Built with: `riscv64-unknown-elf-gcc -march=rv64gc -mabi=lp64 -mcmodel=medany -nostartfiles -nostdlib`
3. [iommu_fault_ttyp_read.elf](/uploads/0fe4aecc801f70fa942dce2f3886378b/iommu_fault_ttyp_read.elf)
4. qemu-system-riscv64 \\ -M virt,iommu-sys=on,aia=aplic-imsic \\ -cpu rv64,smstateen=true \\ -m 8G \\ -nographic \\ -device edu,dma_mask=0xFFFFFFFFFFFFFFFF \\ -bios iommu_fault_ttyp_read.elf
## Additional information
The guest program (`tests/iommu_fault_ttyp_read.c`):
1. Registers a single-stage DC (iohgatp = Bare, iosatp = Sv39, no PASID).
2. Leaves IOVA 0x100000 unmapped and issues an EDU RAM-\>EDU DMA, which is a read of that IOVA through the IOMMU, taking a first-stage read page fault.
3. Reads the fault record and inspects CAUSE and TTYP.
## Expected behaviour (per spec)
Spec references:
- RISC-V IOMMU Architecture Specification, Section 4.2, Table 14, Fault record TTYP field encodings: "2 Untranslated read transaction" and "3 Untranslated write/AMO transaction".
- RISC-V IOMMU Architecture Specification, Section 4.2: "If the TTYP is a transaction with an IOVA, the IOVA is reported in iotval."
## Observed behaviour
```
Fault record
CAUSE = 0x0000000D (read page fault)
TTYP = 0x00000003 (UADDR_WR - write)
BUG: read fault recorded with TTYP = write (UADDR_WR).
CAUSE says read (13) but TTYP says write (3); UADDR_RD is never used.
```
CAUSE = 13 (read page fault, first stage) but TTYP = 3 (untranslated write) in the same record.
## Root cause
In `hw/riscv/riscv-iommu.c`, `riscv_iommu_translate()` (master commit de5d8bfd61, lines 1688-1695):
```c
if (fault) {
unsigned ttype = RISCV_IOMMU_FQ_TTYPE_PCIE_ATS_REQ;
if (iotlb->perm & IOMMU_RW) {
ttype = RISCV_IOMMU_FQ_TTYPE_UADDR_WR;
} else if (iotlb->perm & IOMMU_RO) {
ttype = RISCV_IOMMU_FQ_TTYPE_UADDR_RD;
}
riscv_iommu_report_fault(s, ctx, ttype, fault, enable_pid,
iotlb->iova, iotlb->translated_addr);
```
`IOMMU_RO = 1`, `IOMMU_WO = 2`, `IOMMU_RW = 3` (`include/system/memory.h`). `IOMMU_RW` is the bitmask of both permission bits, not a distinct value. For a read, `iotlb->perm` is `IOMMU_RO` (1), and `1 & IOMMU_RW` (1 & 3) is non-zero, so `ttype` is set to `UADDR_WR`. The `else if (iotlb->perm & IOMMU_RO)` branch that would set `UADDR_RD` is unreachable for any non-zero perm.
The CAUSE, by contrast, is selected with `iotlb->perm & IOMMU_WO` (e.g. lines 431-439 and 549), which correctly distinguishes read from write. That is why CAUSE reports read while TTYP reports write.
## Notes
- Found while checking the fault-record fields for a two-stage second-stage (G-stage) fault. In that scenario iotval (= IOVA) and iotval2 (= the faulting guest physical address) are set correctly.
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issue
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