On Wed, Aug 16, 2023 at 04:06:57PM +0800,
Qian Wen <qian....@intel.com> wrote:
> 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.
>
> Signed-off-by: Qian Wen <qian....@intel.com>
> Reviewed-by: Zhao Liu <zhao1....@intel.com>
> ---
> target/i386/cpu.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 97ad229d8b..5c008b9d7e 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -6008,6 +6008,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index,
> uint32_t count,
> uint32_t die_offset;
> uint32_t limit;
> uint32_t signature[3];
> + uint32_t threads_per_socket;
> X86CPUTopoInfo topo_info;
>
> topo_info.dies_per_pkg = env->nr_dies;
> @@ -6049,8 +6050,9 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index,
> uint32_t count,
> *ecx |= CPUID_EXT_OSXSAVE;
> }
> *edx = env->features[FEAT_1_EDX];
> - if (cs->nr_cores * cs->nr_threads > 1) {
> - *ebx |= (cs->nr_cores * cs->nr_threads) << 16;
> + threads_per_socket = cs->nr_cores * cs->nr_threads;
> + if (threads_per_socket > 1) {
> + *ebx |= MIN(threads_per_socket, 255) << 16;
> *edx |= CPUID_HT;
> }
> if (!cpu->enable_pmu) {
> --
> 2.25.1
>
>
Reviewed-by: Isaku Yamahata <isaku.yamah...@intel.com>
--
Isaku Yamahata <isaku.yamah...@linux.intel.com>
