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authorChristian Krinitsin <mail@krinitsin.com>2025-05-30 16:52:07 +0200
committerChristian Krinitsin <mail@krinitsin.com>2025-05-30 16:52:17 +0200
commit9260319e7411ff8281700a532caa436f40120ec4 (patch)
tree2f6bfe5f3458dd49d328d3a9eb508595450adec0 /gitlab/issues_text/target_riscv/host_missing/accel_missing/2763
parent225caa38269323af1bfc2daadff5ec8bd930747f (diff)
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+RISC-V APLIC emulation: interrupt pending state of direct-delivery level-triggered interrupts is wrong after masking
+Description of problem:
+According to the APLIC specification, the interrupt pending state of a level-triggered interrupt in direct delivery mode should always match the (rectified) input signal:
+
+> When an interrupt domain is in direct delivery mode, the pending bit for a level-sensitive source is always just a copy of the rectified input value.
+
+(From Section 4.7 "Precise effects on interrupt-pending bits" of the specification. See also the more detailed paragraph starting with "If the 
+ source mode is Level1 or Level0 and the interrupt domain is configured in direct delivery mode [...]".)
+
+However, **this is not true in Qemu's emulation**. In particular, in some situations, **a level-triggered interrupt in direct delivery mode can be raised even though the rectified input signal is off**.
+Steps to reproduce:
+1. Set `-machine virt,acpi=off,aia=aplic` to use AIA without IMSIC.
+2. Program APLIC to direct delivery. Program some level triggered interrupt (e.g., an interrupt of a PCIe ECAM controller).
+4. Wait until the IRQ is raised by a device (i.e., `claimi` returns the IRQ).
+5. Mask the interrupt by writing to `clrie`.
+6. Clear the interrupt at the device level.
+7. The state of Qemu's APLIC registers is now:
+   ```
+   Rectified input = 0 (correct)
+   Pending = 1 (incorrect)
+   topi = 0 (correct)
+   ```
+
+Furthermore, if `setie` is written to unmask the IRQ in this situation, the IRQ is raised (in `topi` / `claimi`) despite the signal being off.