Re: [Qemu-devel] [QEMU 1.1 PATCH] Expose CPUID leaf 7 only for -cpu host

[Anthony Liguori]

On 05/16/2012 03:58 PM, Eduardo Habkost wrote:
> Description of the bug:
>
> Since QEMU 0.15, the CPUID information on CPUID[EAX=7,ECX=0] is being
> returned unfiltered to the guest, directly from the GET_SUPPORTED_CPUID
> return value.
>
> The problem is that this makes the resulting CPU feature flags
> unpredictable and dependent on the host CPU and kernel version.

But isn't this exactly what you get with -cpu host?  I mean, the whole point is 
that you're getting whatever the best set of flags that can be exposed by the 
hardware.

> This
> breaks live-migration badly if you try to migrate from a host CPU that
> supports some features on that CPUID leaf (running a recent kenrel) to a
> kernel or host CPU that doesn't support it.

This seems like expected behavior to me.

Regards,

Anthony Liguori

> Migration also is incorrect (the virtual CPU changes under the guest's
> feet) if you migrate in the opposite direction (from an old CPU/kernel
> to a new CPU/kernel), but with less serious consequences (guests
> normally query CPUID information only once on boot).
>
> Fortunately, the bug affects only users using cpudefs with level>= 7.
>
> The right behavior should be to explicitly enable those features on
> [cpudef] config sections or on the "-cpu" command-line arguments. Right
> now there is no predefined CPU model on QEMU that has those features:
> the latest Intel model we have is Sandy Bridge.
>
> I would like to get this fixed on 1.1, so I am submitting this patch,
> that enables those features only if "-cpu host" is being used (as we
> don't have any pre-defined CPU model that actually have those features).
> After 1.1 is released, we can make those features properly configurable
> on [cpudef] and -cpu configuration.
>
> One problem is: with this patch, users with the following setup:
> - Running QEMU 1.0;
> - Using a cpudef having level>= 7; and
> - Running a kernel that supports the features on CPUID leaf 7;
> - Running on a CPU that supports some features on CPUID leaf 7.
> won't be able to live-migrate to QEMU 1.1. But for these users
> live-migration is already broken (they can't live-migrate to hosts with
> older CPUs or older kernels, already), I don't see how to avoid this
> problem.
>
> Signed-off-by: Eduardo Habkost<address@hidden>
> ---
>   target-i386/cpu.c |   24 +++++++++++++++++-------
>   target-i386/cpu.h |    2 ++
>   2 files changed, 19 insertions(+), 7 deletions(-)
>
> diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> index 89b4ac7..c75fa3d 100644
> --- a/target-i386/cpu.c
> +++ b/target-i386/cpu.c
> @@ -238,6 +238,8 @@ typedef struct x86_def_t {
>       /* Store the results of Centaur's CPUID instructions */
>       uint32_t ext4_features;
>       uint32_t xlevel2;
> +    /* The feature bits on CPUID[EAX=7,ECX=0].EBX */
> +    uint32_t cpuid_7_0_ebx_features;
>   } x86_def_t;
>
>   #define I486_FEATURES (CPUID_FP87 | CPUID_VME | CPUID_PSE)
> @@ -521,6 +523,13 @@ static int cpu_x86_fill_host(x86_def_t *x86_cpu_def)
>       x86_cpu_def->ext_features = ecx;
>       x86_cpu_def->features = edx;
>
> +    if (x86_cpu_def->level>= 7) {
> +        host_cpuid(0x7, 0,&eax,&ebx,&ecx,&edx);
> +        x86_cpu_def->cpuid_7_0_ebx_features = ebx;
> +    } else {
> +        x86_cpu_def->cpuid_7_0_ebx_features = 0;
> +    }
> +
>       host_cpuid(0x80000000, 0,&eax,&ebx,&ecx,&edx);
>       x86_cpu_def->xlevel = eax;
>
> @@ -1185,6 +1194,7 @@ int cpu_x86_register(X86CPU *cpu, const char *cpu_model)
>       env->cpuid_kvm_features = def->kvm_features;
>       env->cpuid_svm_features = def->svm_features;
>       env->cpuid_ext4_features = def->ext4_features;
> +    env->cpuid_7_0_ebx = def->cpuid_7_0_ebx_features;
>       env->cpuid_xlevel2 = def->xlevel2;
>       object_property_set_int(OBJECT(cpu), (int64_t)def->tsc_khz * 1000,
>                               "tsc-frequency",&error);
> @@ -1451,13 +1461,13 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
> uint32_t count,
>           *edx = 0;
>           break;
>       case 7:
> -        if (kvm_enabled()) {
> -            KVMState *s = env->kvm_state;
> -
> -            *eax = kvm_arch_get_supported_cpuid(s, 0x7, count, R_EAX);
> -            *ebx = kvm_arch_get_supported_cpuid(s, 0x7, count, R_EBX);
> -            *ecx = kvm_arch_get_supported_cpuid(s, 0x7, count, R_ECX);
> -            *edx = kvm_arch_get_supported_cpuid(s, 0x7, count, R_EDX);
> +        /* Structured Extended Feature Flags Enumeration Leaf
> +         */
> +        if (count == 0) {
> +            *eax = 0; /* Maximum ECX value for sub-leaves */
> +            *ebx = env->cpuid_7_0_ebx; /* Feature flags */
> +            *ecx = 0; /* Reserved */
> +            *edx = 0; /* Reserved */
>           } else {
>               *eax = 0;
>               *ebx = 0;
> diff --git a/target-i386/cpu.h b/target-i386/cpu.h
> index b5b9a50..2460f63 100644
> --- a/target-i386/cpu.h
> +++ b/target-i386/cpu.h
> @@ -741,6 +741,8 @@ typedef struct CPUX86State {
>       /* Store the results of Centaur's CPUID instructions */
>       uint32_t cpuid_xlevel2;
>       uint32_t cpuid_ext4_features;
> +    /* Flags from CPUID[EAX=7,ECX=0].EBX */
> +    uint32_t cpuid_7_0_ebx;
>
>       /* MTRRs */
>       uint64_t mtrr_fixed[11];