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

[Gregory Price <gregory.price@memverge.com>]

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