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i386/cpu: Fix cpu number overflow in CPUID.01H.EBX[23:16]

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The legacy topology enumerated by CPUID.1.EBX[23:16] is defined in SDM
Vol2:

Bits 23-16: Maximum number of addressable IDs for logical processors in
this physical package.

When threads_per_socket > 255, it will 1) overwrite bits[31:24] which is
apic_id, 2) bits [23:16] get truncated.

Specifically, if launching the VM with -smp 256, the value written to
EBX[23:16] is 0 because of data overflow. If the guest only supports
legacy topology, without V2 Extended Topology enumerated by CPUID.0x1f
or Extended Topology enumerated by CPUID.0x0b to support over 255 CPUs,
the return of the kernel invoking cpu_smt_allowed() is false and APs
(application processors) will fail to bring up. Then only CPU 0 is online,
and others are offline.

For example, launch VM via:
qemu-system-x86_64 -M q35,accel=kvm,kernel-irqchip=split \
    -cpu qemu64,cpuid-0xb=off -smp 256 -m 32G \
    -drive file=guest.img,if=none,id=virtio-disk0,format=raw \
    -device virtio-blk-pci,drive=virtio-disk0,bootindex=1 --nographic

The guest shows:
    CPU(s):               256
    On-line CPU(s) list:  0
    Off-line CPU(s) list: 1-255

To avoid this issue caused by overflow, limit the max value written to
EBX[23:16] to 255 as the HW does.

Cc: qemu-stable@nongnu.org
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Qian Wen <qian.wen@intel.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250714080859.1960104-6-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit a62fef5829)
(Mjt: fixup for 10.0.x series due to missing v10.0.0-2217-gf985a1195b
 "i386/cpu: Fix number of addressable IDs field for CPUID.01H.EBX[23:16]")
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
This commit is contained in:
Michael Tokarev 2025-07-17 06:23:26 +03:00
parent 2df7954daa
commit d097553158
1 changed files with 2 additions and 1 deletions
Download patch file Download diff file Expand all files Collapse all files

3
target/i386/cpu.c
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@ -6828,7 +6828,8 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
}
*edx = env->features[FEAT_1_EDX];
if (threads_per_pkg > 1) {
*ebx |= threads_per_pkg << 16;
/* Fixup overflow: max value for bits 23-16 is 255. */
*ebx |= MIN(threads_per_pkg, 255) << 16;
}
if (!cpu->enable_pmu) {
*ecx &= ~CPUID_EXT_PDCM;