Re: [EXT] Re: Concept of LD-ID in QEMU

[Jonathan Cameron]

On Thu, 8 Jun 2023 23:38:34 +0000
Shesha Bhushan Sreenivasamurthy <sheshas@marvell.com> wrote:

> Hi,
> 
> Thinking a bit more, LD in CXL are PCIe endpoint functions. Therefore 1-1 
> mapping of cxl-i2c device per PCIe device is sufficient, and we use function 
> number in BDF as the LD-ID. Does it makes sense ?

LDs are PCIe endpoint functions (always function 0) as seen from the Virtual
Heirarchies (as they end up under a particular vPBB - which look like a
downstream port of a switch to the host) but they aren't from a more general
topology point of view of actual fabric topology and when we are tunneling we
address them via physical port, not virtual port I think (not read that bit
of the spec for a while). See figure 7-23 in CXL 3.0

Outer tunneling command targets a port number (unwrapped at the switch),
inner one targets the LD - unwrapped at the MLD and sent to appropriate LD
including FM owned LD (if I understand this stack directly).

Also no relationship between BDF and LD-ID so don't do that as the maximum
ID is only 16 which would rather limit your PCI toplogies if that's the BDF as
well.

For now just do what you originally said and add an ID (starting from 0).
We can probably do that automatically once more infrastructure is in place.

Jonathan


> 
> From: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
> Sent: Thursday, June 8, 2023 3:36 AM
> To: Shesha Bhushan Sreenivasamurthy <sheshas@marvell.com>
> Cc: linux-cxl@vger.kernel.org <linux-cxl@vger.kernel.org>; 
> <"qemu-devel@nongnu.org" <qemu-devel@nongnu.org>
> Subject: [EXT] Re: Concept of LD-ID in QEMU 
>  
> External Email
> 
> ----------------------------------------------------------------------
> 
> Shesha,
> 
> You've sent an email with the 'In-reply-to' set to one of Terry's patches.
> Please check why that happened and make sure you don't do that in future as
> it hides your unrelated thread in email clients and the archives!
> 
> See
> https://urldefense.proofpoint.com/v2/url?u=https-3A__lore.kernel.org_linux-2Dcxl_20230607221651.2454764-2D1-2Dterry.bowman-40amd.com_T_-23t&d=DwIFAw&c=nKjWec2b6R0mOyPaz7xtfQ&r=Zta64bwn4nurTRpD4LY2OGr8KklkMRPn7Z_Qy0o4unU&m=ZMaAF9tkNCKfk8gYXMiPERZeIhavaQ7MdKtqCbShRF6w5ISrgHqAl6XDOONYbprZ&s=ERVX40JlAgnvRvPLm8sYZalsYDbpUU7YAqI0Ol0faPI&e=
>   
> for example
> 
> ss - Apologies. Will be careful.
> 
> Jonathan
> 
> On Thu, 8 Jun 2023 11:31:53 +0100
> Jonathan Cameron <Jonathan.Cameron@Huawei.com> wrote:
> 
> > On Wed, 7 Jun 2023 23:01:11 +0000
> > Shesha Bhushan Sreenivasamurthy <sheshas@marvell.com> wrote:
> >   
> > > Hi,
> > > For DCD sideband there needs to be LD-ID. Is the following approach 
> > > acceptable?    
> > 
> > QEMU question so +CC qemu-devel
> >   
> > > 
> > >  -device 
> > > cxl-type3,bus=swport0,volatile-memdev=vmem0,dc-memdev=vmem1,id=cxl-vmem0,num-dc-regions=2,ldid=0
> > >  \
> > >  -device 
> > > cxl-type3,bus=swport0,volatile-memdev=vmem1,dc-memdev=vmem2,id=cxl-vmem1,num-dc-regions=2,ldid=1
> > >  \    
> > 
> > Those will be PCI functions at this level so you can't do this until we 
> > have more advanced switch support
> > (it has to know about multiple VHs - right now we only support fixed config 
> > switches).  You could connect them
> > to different switch ports - effectively that will be what it looks like 
> > when we do emulate a configurable switch.
> >   
> > >  -device 
> > > i2c_mctp_cxl,bus=aspeed.i2c.bus.0,address=24,target=cxl-vmem0,cxl-vmem1")
> > > 
> > > With this configuration, the same i2c device is handing both LDs and in 
> > > FMAPI commands we use the LDID specified above.    
> > 
> > This effectively becomes a partial implementation of either an MLD or an 
> > MH-SLD
> > To manage the actual memory access, those will almost certainly need a 
> > bunch of other shared
> > infrastructure.  So I'd ultimately expect the i2c_mctp_cxl device to target 
> > whatever
> > device represents that shared infrastructure - it might be a separate 
> > device or a 'lead' type 3 device.
> > 
> > So I'm not sure how this will fit together longer term.  We need same 
> > infrastructure
> > to work for a mailbox CCI on a MH-SLD/MLD as well and in that case there 
> > isn't a separate
> > device to which we can provide multiple targets as you've done in your 
> > proposal here.
> > 
> > So I think we need to work out how to handle all of (I've probably 
> > forgotten something)
> > X marks done or in progress.
> > 
> > X 1) i2c_mctp_cxl to an SLD (no PCI Mailbox definition for this one)
> >   2) i2c_mctp_cxl directly to an MLD (your case)
> > X 3) i2c_mctp_cxl to a fixed config switch (single fixed VH no MLD capable 
> > ports)
> > X 4) PCI mailbox via switch CCI device that fixed config switch (no MLD 
> > capable ports)
> >        Even with this simple design some fun things you can do.
> >   5) i2c_mctp_cxl to a configurable switch (probably a separate as yet to 
> >be defined management interface - that messes with hotplug)
> >   6) PCI mailbox via switch CCI to configurable switch (again to defined 
> >management interface).
> >   7) i2c_mctp_cxl to an MH-SLD - probably to which ever device also has 
> >support for
> >      tunneling to the FM owned LD via the PCI mailbox.
> > X 8) PCI mailbox on MH-SLD tunneling to the FM owned LD.
> >   9) i2c_mctp_cxl to an MH-MLD - similar to above - this one isn't that 
> >much more complex than MH-SLD case.
> > X 10) PCI mailbox to MH-MLD - similar to above.
> >   11) Tunneling via the switch CCI (then over PCI-VDM - though that detail 
> >isn't visible in QEMU) to an SLD
> >   12) Tunneling via the switch CCI (then PCI-VDM) to an MH-SLD and on to he 
> >FM owned LD.
> >   13) Tunneling via the switch CCI (then over PCI-VDM) to an MLD / MH-MLD
> >  
> > Current i2c_mctp_cxl covers 1 and 3
> > I'm part way through the tunnelling support for (8 and 100) - need to 
> > revisit and wire up the switch CCI PoC
> > properly which will give us 4.
> > 
> > 2 needs MLD support in general which we could maybe make work with a static 
> > binding in a switch but that
> >   would be odd - so we probably need to emulate a configurable switch for 
> >that
> > 5,6 need configurable switch
> > 7 needs same as 2 plus tunneling part similar to 4
> > 9 again probably configurable switch for the MLD part to make sense
> > 11 is fairly straight forward - but not done yet.
> > 12 also not too bad, but needs the MH-SLD part to be fleshed out (some work 
> > on going )
> > 13 needs pretty much everything defined.
> > 
> > Trying to get the command line interface and device model right until we 
> > have PoC code
> > for a few more cases is going to be at most a draft of what it might look 
> > like.
> > 
> > So in short, lots to do.  For now feel free to hack whatever you need in to 
> > be able
> > to test the FM-API side of things, we can move that towards a clean command 
> > line definition
> > once we have one figured out!
> > 
> > Jonathan
> > 
> >   
> > > 
> > > Thanks,
> > > Shesha.    
> > 
> >