Re: [PATCH v3] i386/cpu: fixup number of addressable IDs for processor cores in the physical package

[Michael Tokarev]

11.06.2024 06:23, Chuang Xu wrote:
> When QEMU is started with:
> -cpu host,host-cache-info=on,l3-cache=off \
> -smp 2,sockets=1,dies=1,cores=1,threads=2
> Guest can't acquire maximum number of addressable IDs for processor cores in
> the physical package from CPUID[04H].
>
> When creating a CPU topology of 1 core per package, host-cache-info only
> uses the Host's addressable core IDs field (CPUID.04H.EAX[bits 31-26]),
> resulting in a conflict (on the multicore Host) between the Guest core
> topology information in this field and the Guest's actual cores number.
>
> Fix it by removing the unnecessary condition to cover 1 core per package
> case. This is safe because cores_per_pkg will not be 0 and will be at
> least 1.
>
> Fixes: d7caf13b5fcf ("x86: cpu: fixup number of addressable IDs for logical 
> processors sharing cache")
> Signed-off-by: Guixiong Wei <weiguixiong@bytedance.com>
> Signed-off-by: Yipeng Yin <yinyipeng@bytedance.com>
> Signed-off-by: Chuang Xu <xuchuangxclwt@bytedance.com>
> ---
>   target/i386/cpu.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index bc2dceb647..b68f7460db 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -6426,10 +6426,8 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
> uint32_t count,
>               if (*eax & 31) {
>                   int host_vcpus_per_cache = 1 + ((*eax & 0x3FFC000) >> 14);
>   
> -                if (cores_per_pkg > 1) {
> -                    *eax &= ~0xFC000000;
> -                    *eax |= max_core_ids_in_package(&topo_info) << 26;
> -                }
> +                *eax &= ~0xFC000000;
> +                *eax |= max_core_ids_in_package(&topo_info) << 26;
>                   if (host_vcpus_per_cache > threads_per_pkg) {
>                       *eax &= ~0x3FFC000;
>   

In qemu 9.0, the context is a bit different here:


            if (*eax & 31) {
                int host_vcpus_per_cache = 1 + ((*eax & 0x3FFC000) >> 14);
                int vcpus_per_socket = cs->nr_cores * cs->nr_threads;
                if (cs->nr_cores > 1) {
                    *eax &= ~0xFC000000;
                    *eax |= (pow2ceil(cs->nr_cores) - 1) << 26;
                }
                if (host_vcpus_per_cache > vcpus_per_socket) {

Ie, no max_core_ids_in_package(), cores_per_pkg etc, introduced in
v9.0.0-790-gf602eb925a "i386/cpu: Use CPUCacheInfo.share_level to encode
CPUID[4]" and nearby.

Am I right the above change becomes

             if (*eax & 31) {
                 int host_vcpus_per_cache = 1 + ((*eax & 0x3FFC000) >> 14);
                 int vcpus_per_socket = cs->nr_cores * cs->nr_threads;
-                if (cs->nr_cores > 1) {
-                    *eax &= ~0xFC000000;
-                    *eax |= (pow2ceil(cs->nr_cores) - 1) << 26;
-                }
+                *eax &= ~0xFC000000;
+                *eax |= (pow2ceil(cs->nr_cores) - 1) << 26;
                 if (host_vcpus_per_cache > vcpus_per_socket) {
                     *eax &= ~0x3FFC000;
                     *eax |= (pow2ceil(vcpus_per_socket) - 1) << 14;

in 9.0 -- in other words, just remove the nr_cores condition check
and do the *eax assignment unconditionally ?

From the patch description it seems like it is, but I thought I'd
ask anyway :)

Thanks,

/mjt

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