Re: [Qemu-devel] [PATCH 2/2] i386/cpu/hyperv: support over 64 vcpus for windows guests

[Gonglei (Arei)]

> -----Original Message-----
> From: Eduardo Habkost [mailto:address@hidden
> Sent: Tuesday, September 05, 2017 9:17 PM
> To: Gonglei (Arei)
> Cc: address@hidden; address@hidden; address@hidden;
> address@hidden; address@hidden; address@hidden;
> Huangweidong (C)
> Subject: Re: [PATCH 2/2] i386/cpu/hyperv: support over 64 vcpus for windows
> guests
> 
> On Tue, Sep 05, 2017 at 05:30:05PM +0800, Gonglei wrote:
> > Starting with Windows Server 2012 and Windows 8, if
> > CPUID.40000005.EAX contains a value of -1, Windows assumes specific
> > limit to the number of VPs. In this case, Windows Server 2012
> > guest VMs may use more than 64 VPs, up to the maximum supported
> > number of processors applicable to the specific Windows
> > version being used.
> >
> >
> https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/referenc
> e/tlfs
> >
> > For compatibility, Let's introduce a new property for X86CPU,
> > named "hv-cpuid-limits-eax" as Paolo's suggestion, and set it
> > to "on" before machine 2.10.
> >
> > Signed-off-by: Gonglei <address@hidden>
> > ---
> >  include/hw/i386/pc.h |  5 +++++
> >  target/i386/cpu.c    |  1 +
> >  target/i386/cpu.h    |  2 ++
> >  target/i386/kvm.c    | 18 +++++++++++++++++-
> >  4 files changed, 25 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
> > index 8226904..db32e58 100644
> > --- a/include/hw/i386/pc.h
> > +++ b/include/hw/i386/pc.h
> > @@ -371,6 +371,11 @@ bool e820_get_entry(int, uint32_t, uint64_t *,
> uint64_t *);
> >
> >  #define PC_COMPAT_2_10 \
> >      HW_COMPAT_2_10 \
> > +    {\
> > +        .driver   = TYPE_X86_CPU,\
> > +        .property = "hv_cpuid_limits_eax",\
> 
> The property name is hv-cpuid-limits-eax.
> 
Make sense to me.

> > +        .value    = "on",\
> > +    },\
> >
> >  #define PC_COMPAT_2_9 \
> >      HW_COMPAT_2_9 \
> > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> > index 69676e1..0d47bdd 100644
> > --- a/target/i386/cpu.c
> > +++ b/target/i386/cpu.c
> > @@ -4145,6 +4145,7 @@ static Property x86_cpu_properties[] = {
> >                       false),
> >      DEFINE_PROP_BOOL("vmware-cpuid-freq", X86CPU,
> vmware_cpuid_freq, true),
> >      DEFINE_PROP_BOOL("tcg-cpuid", X86CPU, expose_tcg, true),
> > +    DEFINE_PROP_BOOL("hv-cpuid-limits-eax", X86CPU,
> hv_cpuid_limits_eax, false),
> 
> The property name "hv-cpuid-limits-eax" doesn't say anything
> about what it does exactly when set to true.
> 
> What about just making it int32?  e.g.:
> 
>     DEFINE_PROP_INT32("x-hv-max-vps", X86CPU, hv_max_vps, -1)
> [...]
>     {\
>         .driver   = TYPE_X86_CPU,\
>         .property = "x-hv-max-vps",\
>         .value    = "0x40",\
>     },\
> [...]
>     c->function = HYPERV_CPUID_IMPLEMENT_LIMITS;
>     c->eax = cpu->hv_max_vps;
> 
> 
> (The "x-" prefix indicates that the property is not supposed to
> be a stable user interface.)
> 
I thought about this as well.
but actually we can't assure that users set the x-hv-max-vps an invalid value if
we use this method. Do we really need to expose eax directly?

> 
> >      DEFINE_PROP_END_OF_LIST()
> >  };
> >
> > diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> > index 525d35d..f8b455a 100644
> > --- a/target/i386/cpu.h
> > +++ b/target/i386/cpu.h
> > @@ -1282,6 +1282,8 @@ struct X86CPU {
> >      int32_t socket_id;
> >      int32_t core_id;
> >      int32_t thread_id;
> > +
> > +    bool hv_cpuid_limits_eax;
> >  };
> >
> >  static inline X86CPU *x86_env_get_cpu(CPUX86State *env)
> > diff --git a/target/i386/kvm.c b/target/i386/kvm.c
> > index 6db7783..cf6ef96 100644
> > --- a/target/i386/kvm.c
> > +++ b/target/i386/kvm.c
> > @@ -751,7 +751,23 @@ int kvm_arch_init_vcpu(CPUState *cs)
> >
> >          c = &cpuid_data.entries[cpuid_i++];
> >          c->function = HYPERV_CPUID_IMPLEMENT_LIMITS;
> > -        c->eax = 0x40;
> > +
> > +        if (!cpu->hv_cpuid_limits_eax) {
> > +            /*
> > +             * Starting with Windows Server 2012 and Windows 8, if
> > +             * CPUID.40000005.EAX contains a value of -1, Windows
> > +             * assumes specific limit to the number of VPs. In this case,
> > +             * Windows Server 2012 guest VMs may use more than 64
> VPs,
> > +             * up to the maximum supported number of processors
> > +             * applicable to the specific Windows version being used.
> 
> That was a direct quote from a document, so I recommend citing
> the specific document you quoted.  e.g.:
> 
>     /*
>      * From "Requirements for Implementing the Microsoft
>      * Hypervisor Interface":
>      *
> https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/referenc
> e/tlfs
>      *
>      * "Starting with Windows Server 2012 and Windows 8, if
>      * CPUID.40000005.EAX contains a value of -1, Windows assumes
>      * specific limit to the number of VPs. In this case, Windows
>      * Server 2012 guest VMs may use more than 64 VPs, up to the
>      * maximum supported number of processors applicable to the
>      * specific Windows version being used."
>      */
> 
> 
> > +             *
> > +             * https://docs.microsoft.com/en-us/virtualization/
> > +             *    hyper-v-on-windows/reference/tlfs
> 
> IMO a long line is preferable to a broken URL.
> 
Make sense to me.

Thanks,
-Gonglei