On 10/12/24 下午4:21, Xiaoyao Li wrote:
On 10/9/2024 11:56 AM, Chuang Xu wrote:
When QEMU is started with:
-cpu host,migratable=on,host-cache-info=on,l3-cache=off
-smp 180,sockets=2,dies=1,cores=45,threads=2
On Intel platform:
CPUID.01H.EBX[23:16] is defined as "max number of addressable IDs for
logical processors in the physical package".
When executing "cpuid -1 -l 1 -r" in the guest, we obtain a value of
90 for
CPUID.01H.EBX[23:16], whereas the expected value is 128. Additionally,
executing "cpuid -1 -l 4 -r" in the guest yields a value of 63 for
CPUID.04H.EAX[31:26], which matches the expected result.
As (1+CPUID.04H.EAX[31:26]) rounds up to the nearest power-of-2 integer,
we'd beter round up CPUID.01H.EBX[23:16] to the nearest power-of-2
integer too. Otherwise we may encounter unexpected results in guest.
For example, when QEMU is started with CLI above and xtopology is
disabled,
guest kernel 5.15.120 uses
CPUID.01H.EBX[23:16]/(1+CPUID.04H.EAX[31:26]) to
calculate threads-per-core in detect_ht(). Then guest will get
"90/(1+63)=1"
as the result, even though threads-per-core should actually be 2.
It's kernel's bug instead.
In 1.5.3 "Sub ID Extraction Parameters for initial APIC ID" of "Intel
64 Architecture Processor Topology Enumeration" [1], it is
- SMT_Mask_Width = Log2(RoundToNearestPof2(CPUID.1:EBX[23:16])/
(CPUID.(EAX=4,ECX=0):EAX[31:26]) + 1))
The value of CPUID.1:EBX[23:16] needs to be *rounded* to the neartest
power-of-two integer instead of itself being the power-of-two.
This also is consistency with the SDM, where the comment for bit 23-16
of CPUID.1:EBX is:
The nearest power-of-2 integer that is not smaller than EBX[23:16] is
the number of unique initial APIC IDs reserved for addressing
different logical processors in a physical package.
What I read from this is, the nearest power-of-2 integer that is not
smaller than EBX[23:16] is a different thing than EBX[23:16]. i.e.,
Yes, when I read sdm, I also thought it was a kernel bug. But on my
192ht spr host, the value of CPUID.1:EBX[23:16] was indeed rounded up
to the nearest power of 2 by the hardware. After communicating with
Intel technical support staff, we thought that perhaps the description
in sdm
is not so accurate, and rounding up CPUID.1:EBX[23:16] to the power of 2
in qemu may be more in line with the hardware behavior.
- EBX[23:16]: Maximum number of addressable IDs for logical processors
in this physical package
- pow2ceil(EBX[23:16]): the number of unique initial APIC IDs reserved
for addressing different logical processors in a physical package.
[1]
https://cdrdv2-public.intel.com/759067/intel-64-architecture-processor-topology-enumeration.pdf
And on AMD platform:
CPUID.01H.EBX[23:16] is defined as "Logical processor count". Current
result meets our expectation.
So let us round up CPUID.01H.EBX[23:16] to the nearest power-of-2
integer
only for Intel platform to solve the unexpected result.
Reviewed-by: Zhao Liu <zhao1....@intel.com>
Acked-by: Igor Mammedov <imamm...@redhat.com>
Signed-off-by: Guixiong Wei <weiguixi...@bytedance.com>
Signed-off-by: Yipeng Yin <yinyip...@bytedance.com>
Signed-off-by: Chuang Xu <xuchuangxc...@bytedance.com>
---
target/i386/cpu.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index ff227a8c5c..641d4577b0 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -6462,7 +6462,15 @@ 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;
+ /*
+ * AMD requires logical processor count, but Intel needs
maximum
+ * number of addressable IDs for logical processors per
package.
+ */
+ if (cpu->vendor_cpuid_only && IS_AMD_CPU(env)) {
+ *ebx |= threads_per_pkg << 16;
+ } else {
+ *ebx |= 1 << apicid_pkg_offset(&topo_info) << 16;
+ }
*edx |= CPUID_HT;
}
if (!cpu->enable_pmu) {
