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

[David Hildenbrand <david@xxxxxxxxxx>]

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@xxxxxxxxx>
> > >
> > > Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> > > Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> > > ---
> > >    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