Re: [BUG] cxl can not create region

[Jonathan Cameron <Jonathan.Cameron@huawei.com>]

On Fri, 12 Aug 2022 09:03:02 -0700
Dan Williams <dan.j.williams@intel.com> wrote:

> Jonathan Cameron wrote:
> > On Thu, 11 Aug 2022 18:08:57 +0100
> > Jonathan Cameron via <qemu-devel@nongnu.org> wrote:
> >   
> > > On Tue, 9 Aug 2022 17:08:25 +0100
> > > Jonathan Cameron <Jonathan.Cameron@huawei.com> wrote:
> > >   
> > > > On Tue, 9 Aug 2022 21:07:06 +0800
> > > > Bobo WL <lmw.bobo@gmail.com> wrote:
> > > >     
> > > > > Hi Jonathan
> > > > > 
> > > > > Thanks for your reply!
> > > > > 
> > > > > On Mon, Aug 8, 2022 at 8:37 PM Jonathan Cameron
> > > > > <Jonathan.Cameron@huawei.com> wrote:      
> > > > > >
> > > > > > Probably not related to your problem, but there is a disconnect in QEMU /
> > > > > > kernel assumptionsaround the presence of an HDM decoder when a HB only
> > > > > > has a single root port. Spec allows it to be provided or not as an implementation choice.
> > > > > > Kernel assumes it isn't provide. Qemu assumes it is.
> > > > > >
> > > > > > The temporary solution is to throw in a second root port on the HB and not
> > > > > > connect anything to it.  Longer term I may special case this so that the particular
> > > > > > decoder defaults to pass through settings in QEMU if there is only one root port.
> > > > > >        
> > > > > 
> > > > > You are right! After adding an extra HB in qemu, I can create a x1
> > > > > region successfully.
> > > > > But have some errors in Nvdimm:
> > > > > 
> > > > > [   74.925838] Unknown online node for memory at 0x10000000000, assuming node 0
> > > > > [   74.925846] Unknown target node for memory at 0x10000000000, assuming node 0
> > > > > [   74.927470] nd_region region0: nmem0: is disabled, failing probe      
> > > > 
> > > > Ah. I've seen this one, but not chased it down yet.  Was on my todo list to chase
> > > > down. Once I reach this state I can verify the HDM Decode is correct which is what
> > > > I've been using to test (Which wasn't true until earlier this week). 
> > > > I'm currently testing via devmem, more for historical reasons than because it makes
> > > > that much sense anymore.      
> > > 
> > > *embarassed cough*.  We haven't fully hooked the LSA up in qemu yet.
> > > I'd forgotten that was still on the todo list. I don't think it will
> > > be particularly hard to do and will take a look in next few days.
> > > 
> > > Very very indirectly this error is causing a driver probe fail that means that
> > > we hit a code path that has a rather odd looking check on NDD_LABELING.
> > > Should not have gotten near that path though - hence the problem is actually
> > > when we call cxl_pmem_get_config_data() and it returns an error because
> > > we haven't fully connected up the command in QEMU.  
> > 
> > So a least one bug in QEMU. We were not supporting variable length payloads on mailbox
> > inputs (but were on outputs).  That hasn't mattered until we get to LSA writes.
> > We just need to relax condition on the supplied length.
> > 
> > diff --git a/hw/cxl/cxl-mailbox-utils.c b/hw/cxl/cxl-mailbox-utils.c
> > index c352a935c4..fdda9529fe 100644
> > --- a/hw/cxl/cxl-mailbox-utils.c
> > +++ b/hw/cxl/cxl-mailbox-utils.c
> > @@ -510,7 +510,7 @@ void cxl_process_mailbox(CXLDeviceState *cxl_dstate)
> >      cxl_cmd = &cxl_cmd_set[set][cmd];
> >      h = cxl_cmd->handler;
> >      if (h) {
> > -        if (len == cxl_cmd->in) {
> > +        if (len == cxl_cmd->in || !cxl_cmd->in) {
> >              cxl_cmd->payload = cxl_dstate->mbox_reg_state +
> >                  A_CXL_DEV_CMD_PAYLOAD;
> >              ret = (*h)(cxl_cmd, cxl_dstate, &len);
> > 
> > 
> > This lets the nvdimm/region probe fine, but I'm getting some issues with
> > namespace capacity so I'll look at what is causing that next.
> > Unfortunately I'm not that familiar with the driver/nvdimm side of things
> > so it's take a while to figure out what kicks off what!  
> 
> The whirlwind tour is that 'struct nd_region' instances that represent a
> persitent memory address range are composed of one more mappings of
> 'struct nvdimm' objects. The nvdimm object is driven by the dimm driver
> in drivers/nvdimm/dimm.c. That driver is mainly charged with unlocking
> the dimm (if locked) and interrogating the label area to look for
> namespace labels.
> 
> The label command calls are routed to the '->ndctl()' callback that was
> registered when the CXL nvdimm_bus_descriptor was created. That callback
> handles both 'bus' scope calls, currently none for CXL, and per nvdimm
> calls. cxl_pmem_nvdimm_ctl() translates those generic LIBNVDIMM commands
> to CXL commands.
> 
> The 'struct nvdimm' objects that the CXL side registers have the
> NDD_LABELING flag set which means that namespaces need to be explicitly
> created / provisioned from region capacity. Otherwise, if
> drivers/nvdimm/dimm.c does not find a namespace-label-index block then
> the region reverts to label-less mode and a default namespace equal to
> the size of the region is instantiated.
> 
> If you are seeing small mismatches in namespace capacity then it may
> just be the fact that by default 'ndctl create-namespace' results in an
> 'fsdax' mode namespace which just means that it is a block device where
> 1.5% of the capacity is reserved for 'struct page' metadata. You should
> be able to see namespace capacity == region capacity by doing "ndctl
> create-namespace -m raw", and disable DAX operation.

Currently ndctl create-namespace crashes qemu ;)
Which isn't ideal!

> 
> Hope that helps.
Got me looking at the right code. Thanks!

Jonathan