Re: [PATCH v6] i386/cpu: fixup number of addressable IDs for logical processors in the physical package

[Zhao Liu]

> > > 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.
> 
> I think above justification is important. We need to justify our changes
> with the fact and correct reason.
> 
> I somehow agree to set EBX[23:16] to a value of power-of-2, because the
> 1.5.3 "Sub ID Extraction Parameters for initial APIC ID" of "Intel 64
> Architecture Processor Topology Enumeration" spec says
> 
>     CPUID.1:EBX[23:16] represents the maximum number of addressable IDs
>     (initial APIC ID) that can be assigned to logical processors in a
>     physical package. The value may not be the same as the number of
>     logical processors that are present in the hardware of a physical
>     package.
> 
> It uses the word "may not".

IMO, I don't quite understand your confusion regarding this. I've already
explained the meaning of addressable ID, and the spec you referenced also
clarifies its significance. The reason for this modification is not
because of the two words "may not".

Whether it is "be" or "not be" the same as the number of logical
processors, the essence is that due to topology, the actual number of
initial IDs that can be accommodated in the APIC ID may exceed the number
of logical processors.