On Wed, 18 Sep 2024 20:22:26 +0800
Chuang Xu <xuchuangxc...@bytedance.com> wrote:
> Hi, Igor:
>
> On 2024/9/18 下午8:02, Igor Mammedov wrote:
> > On Sat, 14 Sep 2024 19:01:27 +0800
> > Chuang Xu <xuchuangxc...@bytedance.com> 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
> >>
> >> Execute "cpuid -1 -l 1 -r" in guest, we'll get:
> >> eax=0x000806f8 ebx=0x465a0800 ecx=0xfffaba1f edx=0x3fa9fbff
> >> CPUID.01H.EBX[23:16] is 90, while the expected value is 128.
> >>
> >> Execute "cpuid -1 -l 4 -r" in guest, we'll get:
> >> eax=0xfc004121 ebx=0x02c0003f ecx=0x0000003f edx=0x00000000
> >> CPUID.04H.EAX[31:26] is 63, which is 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 | 8 +++++++-
> >> 1 file changed, 7 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> >> index 4c2e6f3a71..24d60ead9e 100644
> >> --- a/target/i386/cpu.c
> >> +++ b/target/i386/cpu.c
> >> @@ -261,6 +261,12 @@ static uint32_t
> >> max_thread_ids_for_cache(X86CPUTopoInfo *topo_info,
> >> return num_ids - 1;
> >> }
> >>
> >> +static uint32_t max_thread_number_in_package(X86CPUTopoInfo *topo_info)
> >> +{
> >> + uint32_t num_threads = 1 << apicid_pkg_offset(topo_info);
> >> + return num_threads;
> >> +}
> >> +
> >> static uint32_t max_core_ids_in_package(X86CPUTopoInfo *topo_info)
> >> {
> >> uint32_t num_cores = 1 << (apicid_pkg_offset(topo_info) -
> >> @@ -6417,7 +6423,7 @@ 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 |= max_thread_number_in_package(&topo_info) << 16;
> > why not use pow2ceil(threads_per_pkg) instead?
>
> I saw in the latest code that calculations of cpuids involving CPU topology
> all use topo_info,
> so in order to maintain consistency in code style, I also used topo_info for
> calculation.
and we end up with a zoo of ways different topo stuff is calculated.
Given we already have threads_per_pkg calculated within the function,
is cleaner/more self-documenting to reuse it with pow2ceil() instead of
adding yet another helper with less than obvious '1 <<
apicid_pkg_offset(topo_info)' math.
> >
> >> *edx |= CPUID_HT;
> >> }
> >> if (!cpu->enable_pmu)
>
