Re: [Qemu-devel] [PATCH] kvm: Enable -cpu option to hide KVM

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 2014-06-02 at 15:09 -0400, Bandan Das wrote:
> Alex Williamson <alex.williamson@redhat.com> writes:
> 
> > The latest Nvidia driver (337.88) specifically checks for KVM as the
> > hypervisor and reports Code 43 for the driver in a Windows guest when
> > found.  Removing or changing the KVM signature is sufficient for the
> > driver to load and work.  This patch adds an option to easily allow
> > the KVM hypervisor signature to be hidden using '-cpu kvm=off'.  We
> > continue to expose KVM via the cpuid value by default.  The state of
> > this option does not supercede or replace -enable-kvm or the accel=kvm
> > machine option.  This only changes the visibility of KVM to the guest
> > and paravirtual features specifically tied to the KVM cpuid.
> >
> > Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> > ---
> >  target-i386/cpu-qom.h |    1 +
> >  target-i386/cpu.c     |    1 +
> >  target-i386/kvm.c     |   28 +++++++++++++++-------------
> >  3 files changed, 17 insertions(+), 13 deletions(-)
> >
> > diff --git a/target-i386/cpu-qom.h b/target-i386/cpu-qom.h
> > index e9b3d57..0808cfc 100644
> > --- a/target-i386/cpu-qom.h
> > +++ b/target-i386/cpu-qom.h
> > @@ -87,6 +87,7 @@ typedef struct X86CPU {
> >      bool hyperv_time;
> >      bool check_cpuid;
> >      bool enforce_cpuid;
> > +    bool expose_kvm;
> >  
> >      /* if true the CPUID code directly forward host cache leaves to the guest */
> >      bool cache_info_passthrough;
> > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > index 042a48d..cbf1d97 100644
> > --- a/target-i386/cpu.c
> > +++ b/target-i386/cpu.c
> > @@ -2792,6 +2792,7 @@ static Property x86_cpu_properties[] = {
> >      DEFINE_PROP_BOOL("hv-time", X86CPU, hyperv_time, false),
> >      DEFINE_PROP_BOOL("check", X86CPU, check_cpuid, false),
> >      DEFINE_PROP_BOOL("enforce", X86CPU, enforce_cpuid, false),
> > +    DEFINE_PROP_BOOL("kvm", X86CPU, expose_kvm, true),
> >      DEFINE_PROP_END_OF_LIST()
> >  };
> >  
> > diff --git a/target-i386/kvm.c b/target-i386/kvm.c
> > index 0d894ef..b2295d2 100644
> > --- a/target-i386/kvm.c
> > +++ b/target-i386/kvm.c
> > @@ -528,23 +528,25 @@ int kvm_arch_init_vcpu(CPUState *cs)
> >          has_msr_hv_hypercall = true;
> >      }
> >  
> > -    memcpy(signature, "KVMKVMKVM\0\0\0", 12);
> > -    c = &cpuid_data.entries[cpuid_i++];
> > -    c->function = KVM_CPUID_SIGNATURE | kvm_base;
> > -    c->eax = 0;
> > -    c->ebx = signature[0];
> > -    c->ecx = signature[1];
> > -    c->edx = signature[2];
> > +    if (cpu->expose_kvm) {
> > +        memcpy(signature, "KVMKVMKVM\0\0\0", 12);
> > +        c = &cpuid_data.entries[cpuid_i++];
> > +        c->function = KVM_CPUID_SIGNATURE | kvm_base;
> > +        c->eax = 0;
> > +        c->ebx = signature[0];
> > +        c->ecx = signature[1];
> > +        c->edx = signature[2];
> >  
> > -    c = &cpuid_data.entries[cpuid_i++];
> > -    c->function = KVM_CPUID_FEATURES | kvm_base;
> > -    c->eax = env->features[FEAT_KVM];
> > +        c = &cpuid_data.entries[cpuid_i++];
> > +        c->function = KVM_CPUID_FEATURES | kvm_base;
> > +        c->eax = env->features[FEAT_KVM];
> >  
> > -    has_msr_async_pf_en = c->eax & (1 << KVM_FEATURE_ASYNC_PF);
> > +        has_msr_async_pf_en = c->eax & (1 << KVM_FEATURE_ASYNC_PF);
> >  
> > -    has_msr_pv_eoi_en = c->eax & (1 << KVM_FEATURE_PV_EOI);
> > +        has_msr_pv_eoi_en = c->eax & (1 << KVM_FEATURE_PV_EOI);
> >  
> > -    has_msr_kvm_steal_time = c->eax & (1 << KVM_FEATURE_STEAL_TIME);
> > +        has_msr_kvm_steal_time = c->eax & (1 << KVM_FEATURE_STEAL_TIME);
> Does this mean that we don't expose these features if kvm=off is 
> specified ?

Correct, the guest would only learn about them via the bits in this
cpuid return value, so it should not be making use of them when kvm=off.
Thanks,

Alex

> 
> > +    }
> >  
> >      cpu_x86_cpuid(env, 0, 0, &limit, &unused, &unused, &unused);
> >