On 02.07.2012, at 16:25, Avi Kivity wrote:
> 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);
I guess it helps to think :).
>
>
>> +}
>
>
>
>> +
>> +
>> +
>> +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?
I don't think so. We should rather make sure we always have cpu descriptions available close to what people out there actually use.
Alex
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
