Re: [PATCH v5 02/10] kvm: Support for querying fd-based stats

[Paolo Bonzini <pbonzini@redhat.com>]

On 6/7/22 19:44, Dr. David Alan Gilbert wrote:

>> +        return NULL;
>> +    }
>> +    descriptors->kvm_stats_header = kvm_stats_header;
>> +    descriptors->kvm_stats_desc = kvm_stats_desc;
>> +    descriptors->ident = g_strdup(ident);
> 
> There's something that confuses me here; you check your set of
> descriptors above to find any with the matching ident, and if you've
> already got it you return it; OK.  Now, if you don't match then you
> read some stats and store it with that ident - but I don't see
> when you read the stats from the fd, what makes it read the stats that
> correspond to 'ident' ?

If you mean why not some other source, each source has a different file 
descriptor:

+    int stats_fd = kvm_vcpu_ioctl(cpu, KVM_GET_STATS_FD, NULL);

but the descriptors are consistent every time KVM_GET_STATS_FD is 
called, so basically "ident" can be used as a cache key.

If you mean how does it access the right stat, here it uses the offset
field in the descriptor

     ret = pread(stats_fd, stats_data, size_data, 
kvm_stats_header->data_offset);
     ...
     for (i = 0; i < kvm_stats_header->num_desc; ++i) {
         uint64_t *stats;
         pdesc = (void *)kvm_stats_desc + i * size_desc;

         /* Add entry to the list */
         stats = (void *)stats_data + pdesc->offset;

Paolo