Re: [Qemu-devel] [PATCH 5/5] target-i386: fix kvm_default_features overwriting kvm_arch_get_supported_cpuid()

[Eduardo Habkost]

On Tue, Feb 05, 2013 at 05:39:24PM +0100, Igor Mammedov wrote:
> ORing kvm_default_features to def->kvm_features might set unsupported
> bits if kvm_arch_get_supported_cpuid() returns less bits set than in
> kvm_default_features. Fix it by removing kvm_default_features and using
> only what kvm_arch_get_supported_cpuid() returns.

This is exactly what we _must not_ do! We can't change CPUID bits in a
machine-type or we will change the guest ABI. We must keep them stable
for a machine-type and:

* Not change if QEMU is upgraded;
* Not change if the host kernel is upgraded;
* Not change if running on different host hardware.

The only way we are allowed to change the guest ABI is on a new
machine-type. This was the whole point of the bug fix at commit
ef8621b1a3, and this patch reintroduces the bug.

If you are starting QEMU in a host that doesn't support the set of KVM
features QEMU is configured to expose (through the command-line or by
choosing a machine-type), it _should_ fail to start if using -cpu
enforce. This is a feature.

We could enable this behavior on the "pc-next" machine, if we create it
one day, but not on the stable machine-types.


> 
> Mark disable_kvm_pv_eoi() to be removed in favor of compat_props
> when available.
> 
> Signed-off-by: Igor Mammedov <address@hidden>
> ---
>  target-i386/cpu.c |   20 ++++++++------------
>  target-i386/cpu.h |    2 ++
>  2 files changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> index 62fdc84..e66362e 100644
> --- a/target-i386/cpu.c
> +++ b/target-i386/cpu.c
> @@ -220,17 +220,12 @@ typedef struct model_features_t {
>  int check_cpuid = 0;
>  int enforce_cpuid = 0;
>  
> -static uint32_t kvm_default_features = (1 << KVM_FEATURE_CLOCKSOURCE) |
> -        (1 << KVM_FEATURE_NOP_IO_DELAY) |
> -        (1 << KVM_FEATURE_CLOCKSOURCE2) |
> -        (1 << KVM_FEATURE_ASYNC_PF) |
> -        (1 << KVM_FEATURE_STEAL_TIME) |
> -        (1 << KVM_FEATURE_PV_EOI) |
> -        (1 << KVM_FEATURE_CLOCKSOURCE_STABLE_BIT);
> -
> +/* TODO: replace it with compat properties when feature bits
> + * are converted into static properties */
> +bool x86_cpu_no_pv_eoi;
>  void disable_kvm_pv_eoi(void)
>  {
> -    kvm_default_features &= ~(1UL << KVM_FEATURE_PV_EOI);
> +    x86_cpu_no_pv_eoi = true;
>  }
>  
>  void host_cpuid(uint32_t function, uint32_t count,
> @@ -2061,9 +2056,6 @@ static void x86_cpu_initfn(Object *obj)
>      env->cpuid_ext3_features = def->ext3_features;
>      object_property_set_int(OBJECT(cpu), def->xlevel, "xlevel", NULL);
>      env->cpuid_kvm_features = def->kvm_features;
> -    if (kvm_enabled()) {
> -        env->cpuid_kvm_features |= kvm_default_features;
> -    }
>      env->cpuid_svm_features = def->svm_features;
>      env->cpuid_ext4_features = def->ext4_features;
>      env->cpuid_7_0_ebx_features = def->cpuid_7_0_ebx_features;
> @@ -2107,6 +2099,10 @@ static void x86_cpu_def_class_init(ObjectClass *oc, 
> void *data)
>           */
>          memcpy(xcc->info.vendor, hostcc->info.vendor,
>                 sizeof(xcc->info.vendor));
> +        xcc->info.kvm_features = xcc->info.kvm_features;
> +        if (x86_cpu_no_pv_eoi) {
> +            xcc->info.kvm_features &= ~(1UL << KVM_FEATURE_PV_EOI);
> +        }
>      }
>  
>      /* Look for specific models that have the QEMU version in .model_id */
> diff --git a/target-i386/cpu.h b/target-i386/cpu.h
> index 11ef942..51f20b6 100644
> --- a/target-i386/cpu.h
> +++ b/target-i386/cpu.h
> @@ -1246,6 +1246,8 @@ void do_smm_enter(CPUX86State *env1);
>  
>  void cpu_report_tpr_access(CPUX86State *env, TPRAccess access);
>  
> +/* TODO: convert to compat_props */
> +extern bool x86_cpu_no_pv_eoi;
>  void disable_kvm_pv_eoi(void);
>  
>  /* Return name of 32-bit register, from a R_* constant */
> -- 
> 1.7.1
> 

-- 
Eduardo