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

[Robert Malz]

[ 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(-)

-- 
2.34.1