On Fri, 20 Sep 2024 02:29:46 +0800 Zhao Liu <zhao1....@intel.com> wrote:
> Hi Chuang and Igor, > > Sorry for late reply, > > On Wed, Sep 18, 2024 at 09:18:15PM +0800, Chuang Xu wrote: > > Date: Wed, 18 Sep 2024 21:18:15 +0800 > > From: Chuang Xu <xuchuangxc...@bytedance.com> > > Subject: [PATCH v3] i386/cpu: fixup number of addressable IDs for logical > > processors in the physical package > > X-Mailer: git-send-email 2.24.3 (Apple Git-128) > > > > 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 > > > > Try to execute "cpuid -1 -l 1 -r" in guest, we'll obtain a value of 90 for > > CPUID.01H.EBX[23:16], while the expected value is 128. And Try to > > execute "cpuid -1 -l 4 -r" in guest, we'll obtain a value of 63 for > > CPUID.04H.EAX[31:26] as expected. > > > > As (1+CPUID.04H.EAX[31:26]) round 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 theads-per-core should actually be 2. > > > > So let us round up CPUID.01H.EBX[23:16] to the nearest power-of-2 integer > > to solve the unexpected result. > > > > 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 | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/target/i386/cpu.c b/target/i386/cpu.c > > index 4c2e6f3a71..3710ae5283 100644 > > --- a/target/i386/cpu.c > > +++ b/target/i386/cpu.c > > @@ -6417,7 +6417,7 @@ 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; > > + *ebx |= pow2ceil(threads_per_pkg) << 16; > > Yes, the fix is right. > > About the "Maximum number of addressable IDs", the commit 88dd4ca06c83 > ("i386/cpu: Use APIC ID info to encode cache topo in CPUID[4]") > introduced the new way to calculate. > > The pow2ceil() works for current SMP topology, but may be wrong on > hybrid topology, as the reason I listed in the commit message: > > > The nearest power-of-2 integer can be calculated by pow2ceil() or by > > using APIC ID offset/width (like L3 topology using 1 << die_offset [3]). > > > But in fact, CPUID.04H:EAX[bits 25:14] and CPUID.04H:EAX[bits 31:26] > > are associated with APIC ID. For example, in linux kernel, the field > > "num_threads_sharing" (Bits 25 - 14) is parsed with APIC ID. And for > > another example, on Alder Lake P, the CPUID.04H:EAX[bits 31:26] is not > > matched with actual core numbers and it's calculated by: > > "(1 << (pkg_offset - core_offset)) - 1". > > Using APIC ID offset to calculate is the hardware's approach, so I tried > to use APIC ID instead of pow2ceil() and replaced all pow2ceil() case. Well, hybrid case needs some more explanation then. 'pow2ceil(threads_per_pkg) << 16' - does exactly what SDM says for CPUID.01H.EBX[23:16] Can you point to a spec that confirms that above is wrong and explain in more details how hybrid case is supposed to work and where it's documented? > > Hi Igor, do you agree? :-) > > Best Regards, > Zhao >