[PATCH v4 0/2] Fix overflow of the max number of IDs for logic processor and core

[PATCH v4 0/2] Fix overflow of the max number of IDs for logic processor and core

[Qian Wen]

CPUID.1.EBX[23:16]: Maximum number of addressable IDs for logical
processors in this physical package.
CPUID.4:EAX[31:26]: Maximum number of addressable IDs for processor cores
in the physical package.

The current qemu code doesn't limit the value written to these two fields.
If the guest has a huge number of cores, APs (application processor) will
fail to bring up and the wrong info will be reported.
According to HW behavior, setting max value written to CPUID.1.EBX[23:16]
to 255, and CPUID.4:EAX[31:26] to 63.

---
Changes v3 -> v4:
  - Add "Reviewed-by" from Isaku and Xiaoyao.
  - Rebase to the v8.1.0.
Changes v2 -> v3:
  - Add patch 2.
  - Revise the commit message and comment to be clearer.
  - Using MIN() for limitation.
Changes v1 -> v2:
  - Revise the commit message and comment to more clearer.
  - Rebased to v8.1.0-rc2.

Qian Wen (2):
  target/i386: Avoid cpu number overflow in legacy topology
  target/i386: Avoid overflow of the cache parameter enumerated by leaf
    4

 target/i386/cpu.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

base-commit:f5fe7c17ac4e309e47e78f0f9761aebc8d2f2c81
-- 
2.25.1

[PATCH v4 1/2] target/i386: Avoid cpu number overflow in legacy topology

[Qian Wen]

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.wen@intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Isaku Yamahata <isaku.yamahata@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 00f913b638..fc0437bdb1 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -6012,6 +6012,7 @@ 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;
@@ -6053,8 +6054,9 @@ 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

[PATCH v4 2/2] target/i386: Avoid overflow of the cache parameter enumerated by leaf 4

[Qian Wen]

According to SDM, CPUID.0x4:EAX[31:26] indicates the Maximum number of
addressable IDs for processor cores in the physical package. If we
launch over 64 cores VM, the 6-bit field will overflow, and the wrong
core_id number will be reported.

Since the HW reports 0x3f when the intel processor has over 64 cores,
limit the max value written to EBX[31:26] to 63, so max num_cores should
be 64.

Signed-off-by: Qian Wen <qian.wen@intel.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Isaku Yamahata <isaku.yamahata@intel.com>
---
 target/i386/cpu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index fc0437bdb1..90fe0a6a46 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -248,7 +248,7 @@ static void encode_cache_cpuid4(CPUCacheInfo *cache,
     *eax = CACHE_TYPE(cache->type) |
            CACHE_LEVEL(cache->level) |
            (cache->self_init ? CACHE_SELF_INIT_LEVEL : 0) |
-           ((num_cores - 1) << 26) |
+           ((MIN(num_cores, 64) - 1) << 26) |
            ((num_apic_ids - 1) << 14);
 
     assert(cache->line_size > 0);
-- 
2.25.1

Re: [PATCH v4 0/2] Fix overflow of the max number of IDs for logic processor and core

[Wen, Qian]

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Kindly ping for any comments.

Thanks,
Qian

On 8/29/2023 12:24 PM, Qian Wen wrote:
> CPUID.1.EBX[23:16]: Maximum number of addressable IDs for logical
> processors in this physical package.
> CPUID.4:EAX[31:26]: Maximum number of addressable IDs for processor cores
> in the physical package.
>
> The current qemu code doesn't limit the value written to these two fields.
> If the guest has a huge number of cores, APs (application processor) will
> fail to bring up and the wrong info will be reported.
> According to HW behavior, setting max value written to CPUID.1.EBX[23:16]
> to 255, and CPUID.4:EAX[31:26] to 63.
>
> ---
> Changes v3 -> v4:
>   - Add "Reviewed-by" from Isaku and Xiaoyao.
>   - Rebase to the v8.1.0.
> Changes v2 -> v3:
>   - Add patch 2.
>   - Revise the commit message and comment to be clearer.
>   - Using MIN() for limitation.
> Changes v1 -> v2:
>   - Revise the commit message and comment to more clearer.
>   - Rebased to v8.1.0-rc2.
>
> Qian Wen (2):
>   target/i386: Avoid cpu number overflow in legacy topology
>   target/i386: Avoid overflow of the cache parameter enumerated by leaf
>     4
>
>  target/i386/cpu.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> base-commit:f5fe7c17ac4e309e47e78f0f9761aebc8d2f2c81

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