On 10.09.25 18:16, Alejandro Vallejo wrote:
On Wed Sep 10, 2025 at 5:02 PM CEST, Jan Beulich wrote:
On 10.09.2025 16:49, Alejandro Vallejo wrote:
CPU hotplug relies on the guest having access to the legacy online CPU
bitmap that QEMU provides at PIO 0xAF00. But PVH guests have no DM, so
this causes the MADT to get corrupted due to spurious modifications of
the "online" flag in MADT entries and the table checksum during the
initial acpica passes.
I don't understand this MADT corruption aspect, which - aiui - is why
there's a Fixes: tag here. The code change itself looks plausible.
Jan
When there's no DM to provide a real and honest online CPU bitmap on PIO 0xAF00
then we get all 1s (because there's no IOREQ server). Which confuses the GPE
handler.
Somehow, the GPE handler is being triggered. Whether this is due to a real SCI
or just it being spuriously executed as part of the initial acpica pass, I don't
know.
Both statements combined means the checksum and online flags in the MADT get
changed after initial parsing making it appear as-if all 128 CPUs were plugged.
This patch makes the checksums be correct after acpica init.
Grygorii noticed the checksum mismatch while validating an ACPI dump on a PVH
Linux system.
Below is "acpidump -r 0xfc000000" from PVH guest (not dom0) for MADT
before/after this patch:
Before:
Firmware Warning (ACPI): Incorrect checksum in table [APIC] - 0x59, should be
0xFFFFFFE3 (20250404/utcksum-208)
APIC @ 0x0000000000000000
0000: 41 50 49 43 52 00 00 00 02 59 58 65 6E 00 00 00 APICR....YXen...
^^ incorrect
0010: 48 56 4D 00 00 00 00 00 00 00 00 00 48 56 4D 4C HVM.........HVML
0020: 00 00 00 00 00 00 E0 FE 00 00 00 00 02 0A 00 00 ................
0030: 02 00 00 00 00 00 01 0C 00 00 00 00 C0 FE 00 00 ................
0040: 00 00 00 08 00 00 01 00 00 00 00 08 01 02 01 00 ................
0050: 00 00
After:
APIC @ 0x0000000000000000
0000: 41 50 49 43 52 00 00 00 02 76 58 65 6E 00 00 00 APICR....vXen...
^^ correct
0010: 48 56 4D 00 00 00 00 00 00 00 00 00 48 56 4D 4C HVM.........HVML
0020: 00 00 00 00 00 00 E0 FE 00 00 00 00 02 0A 00 00 ................
0030: 02 00 00 00 00 00 01 0C 00 00 00 00 C0 FE 00 00 ................
0040: 00 00 00 08 00 00 01 00 00 00 00 08 01 02 01 00 ................
0050: 00 00
--
Best regards,
-grygorii
