Re: [RFC PATCH 08/19] HostMem: Add private property to indicate to use kvm gmem

[David Hildenbrand <david@redhat.com>]

On 02.08.23 10:03, Xiaoyao Li wrote:
> On 8/2/2023 1:21 AM, David Hildenbrand wrote:
>> On 31.07.23 18:21, Xiaoyao Li wrote:
>>> From: Isaku Yamahata <isaku.yamahata@intel.com>
>>>
>>> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
>>> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
>>> ---
>>>    backends/hostmem.c       | 18 ++++++++++++++++++
>>>    include/sysemu/hostmem.h |  2 +-
>>>    qapi/qom.json            |  4 ++++
>>>    3 files changed, 23 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/backends/hostmem.c b/backends/hostmem.c
>>> index 747e7838c031..dbdbb0aafd45 100644
>>> --- a/backends/hostmem.c
>>> +++ b/backends/hostmem.c
>>> @@ -461,6 +461,20 @@ static void
>>> host_memory_backend_set_reserve(Object *o, bool value, Error **errp)
>>>        }
>>>        backend->reserve = value;
>>>    }
>>> +
>>> +static bool host_memory_backend_get_private(Object *o, Error **errp)
>>> +{
>>> +    HostMemoryBackend *backend = MEMORY_BACKEND(o);
>>> +
>>> +    return backend->private;
>>> +}
>>> +
>>> +static void host_memory_backend_set_private(Object *o, bool value,
>>> Error **errp)
>>> +{
>>> +    HostMemoryBackend *backend = MEMORY_BACKEND(o);
>>> +
>>> +    backend->private = value;
>>> +}
>>>    #endif /* CONFIG_LINUX */
>>>    static bool
>>> @@ -541,6 +555,10 @@ host_memory_backend_class_init(ObjectClass *oc,
>>> void *data)
>>>            host_memory_backend_get_reserve,
>>> host_memory_backend_set_reserve);
>>>        object_class_property_set_description(oc, "reserve",
>>>            "Reserve swap space (or huge pages) if applicable");
>>> +    object_class_property_add_bool(oc, "private",
>>> +        host_memory_backend_get_private,
>>> host_memory_backend_set_private);
>>> +    object_class_property_set_description(oc, "private",
>>> +        "Use KVM gmem private memory");
>>>    #endif /* CONFIG_LINUX */
>>>        /*
>>>         * Do not delete/rename option. This option must be considered
>>> stable
>>> diff --git a/include/sysemu/hostmem.h b/include/sysemu/hostmem.h
>>> index 39326f1d4f9c..d88970395618 100644
>>> --- a/include/sysemu/hostmem.h
>>> +++ b/include/sysemu/hostmem.h
>>> @@ -65,7 +65,7 @@ struct HostMemoryBackend {
>>>        /* protected */
>>>        uint64_t size;
>>>        bool merge, dump, use_canonical_path;
>>> -    bool prealloc, is_mapped, share, reserve;
>>> +    bool prealloc, is_mapped, share, reserve, private;
>>>        uint32_t prealloc_threads;
>>>        ThreadContext *prealloc_context;
>>>        DECLARE_BITMAP(host_nodes, MAX_NODES + 1);
>>> diff --git a/qapi/qom.json b/qapi/qom.json
>>> index 7f92ea43e8e1..e0b2044e3d20 100644
>>> --- a/qapi/qom.json
>>> +++ b/qapi/qom.json
>>> @@ -605,6 +605,9 @@
>>>    # @reserve: if true, reserve swap space (or huge pages) if applicable
>>>    #     (default: true) (since 6.1)
>>>    #
>>> +# @private: if true, use KVM gmem private memory
>>> +#           (default: false) (since 8.1)
>>> +#
>>
>> But that's not what any of this does.
>>
>> This patch only adds a property and doesn't even explain what it intends
>> to achieve with that.
>>
>> How will it be used from a user? What will it affect internally? What
>> will it modify in regards of the memory backend?
> 
> How it will be used is in the next patch, patch 09.
> 
> for kvm_x86_sw_protected_vm type VM, it will allocate private gmem with
> KVM ioctl if the memory backend has property "private" on.

It feels wired up the wrong way.

When creating/initializing the memory backend, we should also take care 
of allocating the gmem_fd, for example, by doing some gmem allocation 
callback, ideally *internally* creating the RAM memory region / RAMBlock.

And we should fail if that is impossible (gmem does not apply to the VM) 
or creating the gmem_fd failed for other reason.

Like passing a RAM_GMEM flag to memory_region_init_ram_flags_nomigrate() 
in ram_backend_memory_alloc(), to then handle it internally, failing if 
there is an error.

-- 
Cheers,

David / dhildenb