ACK: [SRU][N, O][PATCH v2 0/1] x86/acpi: Fix LAPIC/x2APIC parsing order

[Masahiro Yamada]

On Fri, Feb 7, 2025 at 7:09 PM Robert Malz <robert.malz at canonical.com> wrote:
>
> [ Impact ]
>
>  * In Kernels with commit ec9aedb2aa1a (6.7) CPUs can be enumerated in the order
>    which violates ACPI specification that states:
>     "OSPM should initialize processors in the order that they appear in the MADT"
>
>  * The problematic commit parses all LAPIC entries before any x2APIC
>    entries, aiming to ignore x2APIC entries with APIC ID < 255 when valid
>    LAPIC entries exist. However, it disrupts the CPU enumeration order on
>    systems where x2APIC entries precede LAPIC entries in the MADT.
>
>  * In scenarios with strict CPU pinning this behavior might introduce
>    performance degradation or hyperthreading related side-channel
>    vulnerabilities
>
>  * Issue has been fixed upstream with commit 0141978ae75
>
> [ Test Plan ]
>
>  * Issue reproduces only in specific ACPI MADT configurations with
>    mixed LAPIC and x2APIC entries. An example of it would be AMD Genoa CPUs.
>
>  * Easiest way to reproduce it locally is to replace x2APIC entry
>    before LAPIC in MADT table.
>    Steps to reproduce (local VM):
>    1. Set up a VM with 288 vCPUs
>     sudo qemu-system-x86_64 -enable-kvm \
>       -M q35,accel=kvm,kernel-irqchip=split \
>       -device intel-iommu,intremap=on \
>       -smp 288,cores=144,threads=1,sockets=2 \
>       -m 20G \
>       -cpu host \
>       -object memory-backend-ram,id=mem0,size=10G \
>       -object memory-backend-ram,id=mem1,size=10G \
>       -numa node,cpus=0-143,nodeid=0,memdev=mem0 \
>       -numa node,cpus=144-287,nodeid=1,memdev=mem1 \
>       -drive file=/var/lib/libvirt/images/ubuntu24.04.qcow2,format=qcow2 \
>       -boot d
>    2. Dump acpi apic/madt tables from VM
>     acpidump > acpidump.dat
>     acpixtract -a acpidump.dat
>     iasl -d apic.dat
>     ----
>     apic.dsl file will be generated.
>    3. Craft apic.dsl and switch order of CPUs
>    4. Compile apic.dsl to aml file
>     iasl -tc apic.dsl
>    5. Inject new table to VM
>     sudo qemu-system-x86_64 -enable-kvm \
>       -M q35,accel=kvm,kernel-irqchip=split \
>       -device intel-iommu,intremap=on \
>       -smp 288,cores=144,threads=1,sockets=2 \
>       -acpitable file=apic.aml \
>       -m 20G \
>       -cpu host \
>       -object memory-backend-ram,id=mem0,size=10G \
>       -object memory-backend-ram,id=mem1,size=10G \
>       -numa node,cpus=0-143,nodeid=0,memdev=mem0 \
>       -numa node,cpus=144-287,nodeid=1,memdev=mem1 \
>       -drive file=/var/lib/libvirt/images/ubuntu24.04.qcow2,format=qcow2 \
>       -boot d
>     Results:
>     Depending on MADT changes. For following example I have moved single x2APIC entry
>     after the first LAPIC.
>     Incorrect enumeration present on 6.8 Kernels:
>     NUMA node(s):                         2
>     NUMA node0 CPU(s):                    0-143
>     NUMA node1 CPU(s):                    144-287
>     Enumeration on 6.8 Kernel with applied fixes:
>     NUMA node(s):                         2
>     NUMA node0 CPU(s):                    0,2-144
>     NUMA node1 CPU(s):                    1,145-287
>
> [ Where problems could occur ]
>
>  * Fix has been tested using alocal VM with a modified MADT table.
>
>  * MADT table can have multiple possible configurations, making it hard to
>    fully test the fix.
>
>  * Potential regression with CPU enumeration can be hard to detect as was in case of
>    patch that introduced the issue.
>
> [ Other Info ]
>
>  * Patch which introduced an issue: ec9aedb2aa1ab7ac420c00b31f5edc5be15ec167
>
>  * Patch fixing the issue: 0141978ae75bd48bac13fca6de131a5071c32011
>
>  * Originally issue has been reported for AMD Genoa CPU with following enumeration:
>   6.5.0
>    NUMA node(s):                       2
>    NUMA node0 CPU(s):                  0-89,180-269
>    NUMA node1 CPU(s):                  90-179,270-359
>   6.8.0
>    NUMA node(s):                         2
>    NUMA node0 CPU(s):                    0-179
>    NUMA node1 CPU(s):                    180-359
>
> v2:
> - Changed PATCH 1/X numbering
> - Moved cherry-picked section on top of Signed-off-by
>
>
> Zhang Rui (1):
>   x86/acpi: Fix LAPIC/x2APIC parsing order
>
>  arch/x86/kernel/acpi/boot.c | 50 +++++++++++++++++++++++++++++++++----
>  1 file changed, 45 insertions(+), 5 deletions(-)


Signed-off-by: Masahiro Yamada <masahiro.yamada at canonical.com>