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

[Eduardo Habkost]

On Wed, Apr 19, 2017 at 04:16:53PM -0400, Pranith Kumar wrote:
> On Wed, Apr 19, 2017 at 4:13 PM, Eduardo Habkost <address@hidden> wrote:
> > 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.
> 
> We assume an AMD cpu for x86 TCG and it prints this warning when an
> smp guest is run with (cores=2,threads=2) argument. The main point of
> this patch is to remove that warning when using TCG.

What is different under TCG? If guests were confused with
threads=2 + vendor=AMD using KVM, what exactly would make them
not confused by threads=2 + vendor=AMD when using TCG?

I just ran a Fedora 25 guest using "-machine accel=tcg
-smp 2,threads=2", and it still thinks it's running on a 2-core
CPU instead of a 2-thread CPU when CPU vendor is AMD.

-- 
Eduardo