Re: [PATCH v6 10/12] hw/mem/cxl_type3: Add dpa range validation for accesses to DC regions

[fan]

On Fri, Apr 12, 2024 at 06:54:42PM -0400, Gregory Price wrote:
> On Mon, Mar 25, 2024 at 12:02:28PM -0700, nifan.cxl@gmail.com wrote:
> > From: Fan Ni <fan.ni@samsung.com>
> > 
> > All dpa ranges in the DC regions are invalid to access until an extent
> > covering the range has been added. Add a bitmap for each region to
> > record whether a DC block in the region has been backed by DC extent.
> > For the bitmap, a bit in the bitmap represents a DC block. When a DC
> > extent is added, all the bits of the blocks in the extent will be set,
> > which will be cleared when the extent is released.
> > 
> > Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > Signed-off-by: Fan Ni <fan.ni@samsung.com>
> > ---
> >  hw/cxl/cxl-mailbox-utils.c  |  6 +++
> >  hw/mem/cxl_type3.c          | 76 +++++++++++++++++++++++++++++++++++++
> >  include/hw/cxl/cxl_device.h |  7 ++++
> >  3 files changed, 89 insertions(+)
> > 
> > diff --git a/hw/cxl/cxl-mailbox-utils.c b/hw/cxl/cxl-mailbox-utils.c
> > index 7094e007b9..a0d2239176 100644
> > --- a/hw/cxl/cxl-mailbox-utils.c
> > +++ b/hw/cxl/cxl-mailbox-utils.c
> > @@ -1620,6 +1620,7 @@ static CXLRetCode cmd_dcd_add_dyn_cap_rsp(const 
> > struct cxl_cmd *cmd,
> >  
> >          cxl_insert_extent_to_extent_list(extent_list, dpa, len, NULL, 0);
> >          ct3d->dc.total_extent_count += 1;
> > +        ct3_set_region_block_backed(ct3d, dpa, len);
> >  
> >          ent = QTAILQ_FIRST(&ct3d->dc.extents_pending);
> >          cxl_remove_extent_from_extent_list(&ct3d->dc.extents_pending, ent);
> 
> while looking at the MHD code, we had decided to "reserve" the blocks in
> the bitmap in the call to `qmp_cxl_process_dynamic_capacity` in order to
> prevent a potential double-allocation (basically we need to sanity check
> that two hosts aren't reserving the region PRIOR to the host being
> notified).
> 
> I did not see any checks in the `qmp_cxl_process_dynamic_capacity` path
> to prevent pending extents from being double-allocated.  Is this an
> explicit choice?
> 
> I can see, for example, why you may want to allow the following in the
> pending list: [Add X, Remove X, Add X].  I just want to know if this is
> intentional or not. If not, you may consider adding a pending check
> during the sanity check phase of `qmp_cxl_process_dynamic_capacity`
> 
> ~Gregory

First, for remove request, pending list is not involved. See cxl r3.1,
9.13.3.3. Pending basically means "pending to add". 
So for the above example, in the pending list, you can see [Add x, add x] if the
event is not processed in time.
Second, from the spec, I cannot find any text saying we cannot issue
another add extent X if it is still pending.
>From the kernel side, if the first one is accepted, the second one will
get rejected, and there is no issue there.
If the first is reject for some reason, the second one can get
accepted or rejected and do not need to worry about the first one.


Fan