Re: [Qemu-devel] [PATCH] tcg/i386: Do not display HT warning for TCG

[Eduardo Habkost]

On Wed, Apr 19, 2017 at 04:00:49PM -0400, Pranith Kumar wrote:
> On Wed, Apr 19, 2017 at 3:54 PM, Pranith Kumar <address@hidden> wrote:
> > When we enable hyperthreading (using threads smp argument), we warn
> > the user if the cpu is an AMD cpu. This does not make sense on TCG and
> > is also obsolete now that AMD Ryzen support hyperthreading.
> >
> > Fix this by adding CPUID_HT bit to the TCG features and explicitly
> > checking this bit in the cpuid.
> >
> > Signed-off-by: Pranith Kumar <address@hidden>
> > ---
> >  target/i386/cpu.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> > index 13c0985f11..f34bb5ead7 100644
> > --- a/target/i386/cpu.c
> > +++ b/target/i386/cpu.c
> > @@ -202,12 +202,12 @@ static void x86_cpu_vendor_words2str(char *dst, 
> > uint32_t vendor1,
> >  #define TCG_FEATURES (CPUID_FP87 | CPUID_PSE | CPUID_TSC | CPUID_MSR | \
> >            CPUID_PAE | CPUID_MCE | CPUID_CX8 | CPUID_APIC | CPUID_SEP | \
> >            CPUID_MTRR | CPUID_PGE | CPUID_MCA | CPUID_CMOV | CPUID_PAT | \
> > -          CPUID_PSE36 | CPUID_CLFLUSH | CPUID_ACPI | CPUID_MMX | \
> > +          CPUID_PSE36 | CPUID_CLFLUSH | CPUID_ACPI | CPUID_MMX | CPUID_HT 
> > | \
> >            CPUID_FXSR | CPUID_SSE | CPUID_SSE2 | CPUID_SS | CPUID_DE)
> >            /* partly implemented:
> >            CPUID_MTRR, CPUID_MCA, CPUID_CLFLUSH (needed for Win64) */
> >            /* missing:
> > -          CPUID_VME, CPUID_DTS, CPUID_SS, CPUID_HT, CPUID_TM, CPUID_PBE */
> > +          CPUID_VME, CPUID_DTS, CPUID_SS, CPUID_TM, CPUID_PBE */
> >  #define TCG_EXT_FEATURES (CPUID_EXT_SSE3 | CPUID_EXT_PCLMULQDQ | \
> >            CPUID_EXT_MONITOR | CPUID_EXT_SSSE3 | CPUID_EXT_CX16 | \
> >            CPUID_EXT_SSE41 | CPUID_EXT_SSE42 | CPUID_EXT_POPCNT | \
> > @@ -3643,7 +3643,7 @@ static void x86_cpu_realizefn(DeviceState *dev, Error 
> > **errp)
> >
> >      qemu_init_vcpu(cs);
> >
> > -    /* Only Intel CPUs support hyperthreading. Even though QEMU fixes this
> > +    /* Only some CPUs support hyperthreading. Even though QEMU fixes this
> >       * issue by adjusting CPUID_0000_0001_EBX and CPUID_8000_0008_ECX
> >       * based on inputs (sockets,cores,threads), it is still better to gives
> >       * users a warning.
> > @@ -3651,8 +3651,8 @@ static void x86_cpu_realizefn(DeviceState *dev, Error 
> > **errp)
> >       * NOTE: the following code has to follow qemu_init_vcpu(). Otherwise
> >       * cs->nr_threads hasn't be populated yet and the checking is 
> > incorrect.
> >       */
> > -    if (!IS_INTEL_CPU(env) && cs->nr_threads > 1 && !ht_warned) {
> > -        error_report("AMD CPU doesn't support hyperthreading. Please 
> > configure"
> > +    if ((env->features[FEAT_1_EDX] & CPUID_HT) && (cs->nr_threads > 1) && 
> > !ht_warned) {
> 
> I missed a '!' here. We need to warn if CPUID_HT is not set. But I see
> that it is not being set even on HT enabled processors (i7-3770). How
> do I test for it?

CPUID_HT is automatically set on the CPU if
(nr_threads * nr_cores) > 1. See the "case 1:" block in
cpu_x86_cpuid().

AFAIK, the point of the warning is to let the user know that the
guest OS is likely to ignore thread topology information if CPUID
vendor is not Intel, and has nothing to do with TCG or KVM
capabilities. Maybe the warning is obsolete today and guests
won't be confused by a HT AMD CPU, but we need to confirm that.

-- 
Eduardo