Re: [Qemu-devel] [PATCH v2 1/3] KVM: Add new -cpu best

[Avi Kivity]

On 06/26/2012 07:39 PM, Alexander Graf wrote:
> During discussions on whether to make -cpu host the default in SLE, I found
> myself disagreeing to the thought, because it potentially opens a big can
> of worms for potential bugs. But if I already am so opposed to it for SLE, how
> can it possibly be reasonable to default to -cpu host in upstream QEMU? And
> what would a sane default look like?
> 
> So I had this idea of looping through all available CPU definitions. We can
> pretty well tell if our host is able to execute any of them by checking the
> respective flags and seeing if our host has all features the CPU definition
> requires. With that, we can create a -cpu type that would fall back to the
> "best known CPU definition" that our host can fulfill. On my Phenom II
> system for example, that would be -cpu phenom.
> 
> With this approach we can test and verify that CPU types actually work at
> any random user setup, because we can always verify that all the -cpu types
> we ship actually work. And we only default to some clever mechanism that
> chooses from one of these.
> 
>  
> +/* Are all guest feature bits present on the host? */
> +static bool cpu_x86_feature_subset(uint32_t host, uint32_t guest)
> +{
> +    int i;
> +
> +    for (i = 0; i < 32; i++) {
> +        uint32_t mask = 1 << i;
> +        if ((guest & mask) && !(host & mask)) {
> +            return false;
> +        }
> +    }
> +
> +    return true;

    return !(guest & ~host);


> +}



> +
> +
> +
> +static void cpu_x86_fill_best(x86_def_t *x86_cpu_def)
> +{
> +    x86_def_t *def;
> +
> +    x86_cpu_def->family = 0;
> +    x86_cpu_def->model = 0;
> +    for (def = x86_defs; def; def = def->next) {
> +        if (cpu_x86_fits_host(def) && cpu_x86_fits_higher(def, x86_cpu_def)) 
> {
> +            memcpy(x86_cpu_def, def, sizeof(*def));
> +        }
      *x86_cpu_def = *def;
> +    }
> +
> +    if (!x86_cpu_def->family && !x86_cpu_def->model) {
> +        fprintf(stderr, "No fitting CPU model found!\n");
> +        exit(1);
> +    }
> +}
> +
>  static int unavailable_host_feature(struct model_features_t *f, uint32_t 
> mask)
>  {
>      int i;
> @@ -878,6 +957,8 @@ static int cpu_x86_find_by_name(x86_def_t *x86_cpu_def, 
> const char *cpu_model)
>              break;
>      if (kvm_enabled() && name && strcmp(name, "host") == 0) {
>          cpu_x86_fill_host(x86_cpu_def);
> +    } else if (kvm_enabled() && name && strcmp(name, "best") == 0) {
> +        cpu_x86_fill_best(x86_cpu_def);
>      } else if (!def) {
>          goto error;
>      } else {
> 

Should we copy the cache size etc. from the host?


-- 
error compiling committee.c: too many arguments to function