Re: [PATCH v2] target/i386: Avoid cpu number overflow in legacy topology

[Igor Mammedov <imammedo@redhat.com>]

On Wed,  9 Aug 2023 18:27:32 +0800
Qian Wen <qian.wen@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 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 AP's bring-up will
> fail. Then only CPU 0 is online, and others are offline.
> 
> To avoid this issue caused by overflow, limit the max value written to
> EBX[23:16] to 255.

what happens on real hw or in lack of thereof what SDM says about this
value when there is more than 255 threads?.


> Signed-off-by: Qian Wen <qian.wen@intel.com>
> ---
> Changes v1 -> v2:
>  - Revise the commit message and comment to more clearer.
>  - Rebased to v8.1.0-rc2.
> ---
>  target/i386/cpu.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 97ad229d8b..6e1d88fbd7 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 +6050,19 @@ 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;
> +        /*
> +         * Only bits [23:16] represent the maximum number of addressable
> +         * IDs for logical processors in this physical package.
> +         * When thread_per_socket > 255, it will 1) overwrite bits[31:24]
> +         * which is apic_id, 2) bits [23:16] get truncated.
> +         */
> +        threads_per_socket = cs->nr_cores * cs->nr_threads;
> +        if (threads_per_socket > 255) {
> +            threads_per_socket = 255;
> +        }
> +
> +        if (threads_per_socket > 1) {
> +            *ebx |= threads_per_socket << 16;
>              *edx |= CPUID_HT;
>          }
>          if (!cpu->enable_pmu) {