Re: CXL Namespaces of ACPI disappearing in Qemu demo

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

On Tue, 5 Sep 2023 18:45:02 +0800
Yuquan Wang <wangyuquan1236@phytium.com.cn> wrote:

> Hi, Jonathan
> On 2023-09-04 20:43,  jonathan.cameron wrote:
> > 
> > At the system design level, MMIO space of Root complex register space via RCRB
> > does not map in a similar fashion to PCIE MMIO space (which is handled via
> > address decoding in the PCIE fabric). It is much more similar to MMIO for platform
> > devices - as such the implementation handles in like a platform device (well 16 of
> > them which seemed enough for any sane usecase).
> > 
> >   
> 
> Oh,thanks! According to above, therefore, the core factor is the implementation of RCRB.
> 
> > 
> > So in theory we could make some space for the CXL root bridge RCRB registers
> > but it would make various generic paths more complex.  In a real system
> > those registers are likely to be far from the PCI MMIO space anyway so the
> > way it's modeled is probably more realistic than pushing the RCRB into the
> > existing allocation.
> >   
> 
> Here implies that all CXL root bridge will use RCRB registers.
> 
> From Table 8-17 and Figure 9-14 in CXL3.0 specification, I understood that only RCH DP &
> RCD UP will use RCRBs, and CXL host bridges VH mode will use other way to realize
> the CHBCR. I had tried to find more explanation in CXL spec, but I haven't found. Hence 
> this is why I am confused.

Ah. That distinction is a bit messy.  Both an RCRB and CHBCR (CXL Host Bridge Component
Registers) are similar in the sense that they are mapped in host memory space. 
As I understand it the distinction is more about the format / contents of that memory
than how you access them. As an aside, they are described by a static ACPI table,
so they can't be in the MMIO space used for BARs etc.

Jonathan


> 
> Many thanks
> Yuquan