[Libguestfs] [PATCH v2v] convert: Allow preferred block driver to be specified on the command line

[Laszlo Ersek]

On 3/7/23 09:35, Richard W.M. Jones wrote:
> On Tue, Mar 07, 2023 at 08:48:56AM +0100, Laszlo Ersek wrote:
>> On 3/6/23 17:52, Richard W.M. Jones wrote:
>>
>>> diff --git a/convert/convert_windows.ml b/convert/convert_windows.ml
>>> index 9d8d271d05..f36b486359 100644
>>> --- a/convert/convert_windows.ml
>>> +++ b/convert/convert_windows.ml
>>> @@ -38,7 +38,7 @@ module G = Guestfs
>>>   * time the Windows VM is booted on KVM.
>>>   *)
>>>  
>>> -let convert (g : G.guestfs) _ inspect i_firmware _ static_ips =
>>> +let convert (g : G.guestfs) _ inspect i_firmware block_driver _ static_ips =
>>>    (*----------------------------------------------------------------------*)
>>>    (* Inspect the Windows guest. *)
>>>  
>>> @@ -47,6 +47,16 @@ let convert (g : G.guestfs) _ inspect i_firmware _ static_ips =
>>>     *)
>>>    let virtio_win =
>>>      Inject_virtio_win.from_environment g inspect.i_root Config.datadir in
>>> +  (match block_driver with
>>> +   | Virtio_blk -> () (* the default, no need to do anything *)
>>> +(*
>>> +   | Virtio_scsi ->
>>> +      let drivers = Inject_virtio_win.get_block_driver_priority virtio_win in
>>> +      let drivers = "vioscsi" :: drivers in
>>> +      Inject_virtio_win.set_block_driver_priority virtio_win drivers
>>> +*)
>>> +   | IDE -> assert false (* not possible - but maybe ...? *)
>>> +  );
>>>  
>>>    (* If the Windows guest appears to be using group policy.
>>>     *
>>
>> So ["virtio_blk"; "vrtioblk"; "viostor"] in
>> "common/mlcustomize/inject_virtio_win.ml" would be replaced with a
>> reference cell, and get_block_driver_priority /
>> set_block_driver_priority would manipulate that.
> 
> References are really just structures with mutable fields and some
> syntactic sugar:
> 
> https://github.com/ocaml/ocaml/blob/66db964f48869c6470b8ccd1ed672e244ba9d680/stdlib/stdlib.ml#L237
> 
> The reason mutable fields of structures in general have to be
> specially marked with the "mutable" keyword is because of the
> "remembered set", an obscure corner of generational garbage collection:
> 
> https://www.cs.cornell.edu/courses/cs3110/2012sp/lectures/lec26-gc/lec26.html
> 
> If mutable fields didn't exist then you could never have a pointer
> from the old generation into the new generation, because new
> generation objects are always younger than old generation objects, and
> as all fields have to be set at object creation time (no mutation),
> there cannot exist pointers from old to new.
> 
> Since mutable fields _do_ exist, it's possible for such pointers to
> exist.  When you do a minor GC of the younger generation, to avoid
> having to scan over the whole of the old heap to find these pointers,
> when a mutable field is mutated a record is added to the "remembered
> set", which is treated as an additional set of roots for the minor GC.
> 
> After each minor GC the remembered set can be cleared.

Why is that?

If the "remembered" objects remain in the young generation, still only
referenced by objects of the old generation, then the extra roots
pointing to these young objects should persist, shouldn't they? A young
object can be removed from the remembered set when it transitions to the
old generation at the same time -- but that need not happen at each GC
run over the young generation. IIUC.

> But there's
> still some overhead here, so marking fields in a struct as mutable is
> not cost free.  (Of course in this specific case the cost is
> infinitesimally small, but in general it can be a concern.)
> 
> This is a long way of saying that it's not a reference cell, but a
> plain field in the 't' struct, with extra runtime overhead and code
> generated for when it is mutated.

Thanks!
Laszlo