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

[Jonathan Cameron via <qemu-devel@nongnu.org>]

On Tue, 16 Apr 2024 09:37:09 -0700
fan <nifan.cxl@gmail.com> wrote:

> On Tue, Apr 16, 2024 at 04:00:56PM +0100, Jonathan Cameron wrote:
> > On Mon, 15 Apr 2024 10:37:00 -0700
> > fan <nifan.cxl@gmail.com> wrote:
> >   
> > > 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.  
> > 
> > I think there is text saying that the capacity is not released for reuse
> > by the device until it receives a response from the host.   Whilst
> > it's not explicit on offers to the same host, I'm not sure that matters.
> > So I don't think it is suppose to queue multiple extents...  
> 
> Are you suggesting we add a check here to reject the second add when the
> first one is still pending?

Yes.  The capacity is not back with the device to reissue.
On an MH-MLD/SLD we'd need to prevent it being added (not shared) to multiple hosts,
this is kind of the temporal equivalent of that.

> 
> Currently, we do not allow releasing an extent when it is still pending,
> which aligns with the case you mentioned above "not release for reuse", I
> think.
> Can the second add mean a retry instead of reuse? 
No - or at least the device should not be doing that.  The FM might try
again, but only once it knows try 1 failed. For reasons of this aligning
with MHD case where you definitely can't offer it to more than one host,
I think we should not do it.  Whether we should put any effort into blocking
it is a different question.  User error :)

Note, the host must not remove a log entry until it has dealt with it
(sent a response) so there is no obvious reason to bother with a retry.
Maybe a booting host would reject all offered extents (because it's not ready
for them yet), but then I'd want the FM to explicitly decide to tell the device
to offer gain.

Whilst this is a custom interface, the equivalent FM API does say.

"The command, with selection policy Enable Shared Access, shall also fail with Invalid
Input under the following conditions:
• When the specified region is not Sharable
• When the tagged capacity is already mapped to any Host ID via a non-Sharable
region
• When the tagged capacity cannot be added to the requested region due to deviceimposed
restrictions
• When the same tagged capacity is currently accessible by the same LD"

Little fuzzy because of the whole pending vs 'mapped / accessible' wording but
I think intent is you can't send again until first one is dealt with.

Jonathan

> 
> Fan
> 
> > 
> >   
> > > 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
> > >   
> >