Re: [Qemu-devel] [PATCH v3 2/2] i386: expose "TCGTCGTCGTCG" in the 0x40000000 CPUID leaf

[Eduardo Habkost]

On Tue, May 09, 2017 at 12:20:34PM +0100, Daniel P. Berrange wrote:
> Currently when running KVM, we expose "KVMKVMKVM\0\0\0" in
> the 0x40000000 CPUID leaf. Other hypervisors (VMWare,
> HyperV, Xen, BHyve) all do the same thing, which leaves
> TCG as the odd one out.
> 
> The CPUID signature is used by software to detect which
> virtual environment they are running in and (potentially)
> change behaviour in certain ways. For example, systemd
> supports a ConditionVirtualization= setting in unit files.
> The virt-what command can also report the virt type it is
> running on
> 
> Currently both these apps have to resort to custom hacks
> like looking for 'fw-cfg' entry in the /proc/device-tree
> file to identify TCG.
> 
> This change thus proposes a signature "TCGTCGTCGTCG" to be
> reported when running under TCG.
> 
> To hide this, the -cpu option tcg-cpuid=off can be used.
> 
> Signed-off-by: Daniel P. Berrange <address@hidden>
> ---
>  include/hw/i386/pc.h |  5 +++++
>  target/i386/cpu.c    | 22 ++++++++++++++++++++++
>  target/i386/cpu.h    |  1 +
>  3 files changed, 28 insertions(+)
> 
> diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
> index f278b3a..3aec60f 100644
> --- a/include/hw/i386/pc.h
> +++ b/include/hw/i386/pc.h
> @@ -376,6 +376,11 @@ bool e820_get_entry(int, uint32_t, uint64_t *, uint64_t 
> *);
>  #define PC_COMPAT_2_8 \
>      HW_COMPAT_2_8 \
>      {\
> +        .driver   = TYPE_X86_CPU,\
> +        .property = "tcg-cpuid",\
> +        .value    = "off",\
> +    },\
> +    {\
>          .driver   = "kvmclock",\
>          .property = "x-mach-use-reliable-get-clock",\
>          .value    = "off",\
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 8cb4af4..ee4f515 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -2627,12 +2627,15 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
> uint32_t count,
>      CPUState *cs = CPU(cpu);
>      uint32_t pkg_offset;
>      uint32_t limit;
> +    uint32_t signature[3];
>  
>      /* Calculate & apply limits for different index ranges */
>      if (index >= 0xC0000000) {
>          limit = env->cpuid_xlevel2;
>      } else if (index >= 0x80000000) {
>          limit = env->cpuid_xlevel;
> +    } else if (index & 0x40000000) {

Why did you decide to use (index & 0x40000000) instead of
(index >= 0x40000000)? The latter would be more consistent with
the rest of the code.


> +        limit = 0x40000000;

This is not strictly wrong, but it will make CPUID[0x40000001]
return arbitrary bits (CPUID[env->cpuid_level]). Guests aren't
supposed to look at CPUID[0x40000001] without checking
CPUID[0x40000000] first, but probably it's safer to set
limit = 0x40000001 and return all-zeroes on CPUID[0x40000001],
just in case.

>      } else {
>          limit = env->cpuid_level;
>      }
> @@ -2867,6 +2870,24 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
> uint32_t count,
>          }
>          break;
>      }
> +    case 0x40000000:
> +        /*
> +         * CPUID code in kvm_arch_init_vcpu() ignores stuff
> +         * set here, but we restrict to TCG none the less.
> +         */
> +        if (tcg_enabled() && cpu->expose_tcg) {
> +            memcpy(signature, "TCGTCGTCGTCG", 12);
> +            *eax = 0;

On both KVM and Hyper-V, CPUID[0x40000000].EAX is the maximum
CPUID function. KVM has the additional exception that eax=0 on
the output is interpreted as 0x40000001.

Setting this to 0x40000000 or 0x40000001 would make things more
consistent for guests. Setting it to 0x40000001 would make it
safer (see my other comment above).


> +            *ebx = signature[0];
> +            *ecx = signature[1];
> +            *edx = signature[2];
> +        } else {
> +            *eax = 0;
> +            *ebx = 0;
> +            *ecx = 0;
> +            *edx = 0;
> +        }
> +        break;
>      case 0x80000000:
>          *eax = env->cpuid_xlevel;
>          *ebx = env->cpuid_vendor1;
> @@ -4008,6 +4029,7 @@ static Property x86_cpu_properties[] = {
>      DEFINE_PROP_BOOL("kvm-no-smi-migration", X86CPU, kvm_no_smi_migration,
>                       false),
>      DEFINE_PROP_BOOL("vmware-cpuid-freq", X86CPU, vmware_cpuid_freq, true),
> +    DEFINE_PROP_BOOL("tcg-cpuid", X86CPU, expose_tcg, true),
>      DEFINE_PROP_END_OF_LIST()
>  };
>  
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index c4602ca..c25f0ce 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -1216,6 +1216,7 @@ struct X86CPU {
>      bool check_cpuid;
>      bool enforce_cpuid;
>      bool expose_kvm;
> +    bool expose_tcg;
>      bool migratable;
>      bool max_features; /* Enable all supported features automatically */
>      uint32_t apic_id;
> -- 
> 2.9.3
> 

-- 
Eduardo