Re: [PATCH] hw/mem: support zero memory size CXL device

[Jonathan Cameron]

On Mon,  2 Dec 2024 17:03:11 -0600
Hongjian Fan <hongjian.fan@seagate.com> wrote:

> According to CXL 3.1 specification, 8.1.3.8 DVSEC CXL Range Registers "A 
> CXL.mem-capable device is permitted to report zero memory size."
> This patch will allow a CXL type3 device to be initialized with zero memory 
> size, when there is no memory device property provided ( neither volatile, 
> persistent, nor dynamic region).
Wrap at less than 75 chars.

I'm curious.  Why do you want to emulate such a device?

There are many things the CXL spec allows that we don't emulate.
I'd normally count this as just another one of those.

Jonathan


> 
> Signed-off-by: Hongjian Fan <hongjian.fan@seagate.com>
> ---
>  hw/mem/cxl_type3.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/hw/mem/cxl_type3.c b/hw/mem/cxl_type3.c
> index 5cf754b38f..35caa186ca 100644
> --- a/hw/mem/cxl_type3.c
> +++ b/hw/mem/cxl_type3.c
> @@ -159,7 +159,12 @@ static int ct3_build_cdat_table(CDATSubHeader 
> ***cdat_table, void *priv)
>      int len = 0;
>  
>      if (!ct3d->hostpmem && !ct3d->hostvmem && !ct3d->dc.num_regions) {
> -        return 0;
> +        // zero memory size device. Build one entry with size 0

Comment syntax /* */

> +        table = g_malloc0(CT3_CDAT_NUM_ENTRIES * sizeof(*table));
> +        ct3_build_cdat_entries_for_mr(&(table[0]), dsmad_handle++,
> +                                0, false, false, 0);
> +        *cdat_table = g_steal_pointer(&table);
> +        return CT3_CDAT_NUM_ENTRIES;
>      }
>  
>      if (ct3d->hostvmem) {
> @@ -712,8 +717,11 @@ static bool cxl_setup_memory(CXLType3Dev *ct3d, Error 
> **errp)
>  
>      if (!ct3d->hostmem && !ct3d->hostvmem && !ct3d->hostpmem
>          && !ct3d->dc.num_regions) {
> -        error_setg(errp, "at least one memdev property must be set");
> -        return false;
> +        // no memdev property provided. Default to zero memory size device
> +        ct3d->cxl_dstate.pmem_size = 0;
> +        ct3d->cxl_dstate.vmem_size = 0;
> +        ct3d->cxl_dstate.static_mem_size = 0;
> +        return true;
>      } else if (ct3d->hostmem && ct3d->hostpmem) {
>          error_setg(errp, "[memdev] cannot be used with new "
>                           "[persistent-memdev] property");