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Re: CXL Namespaces of ACPI disappearing in Qemu demo
From: |
Jonathan Cameron |
Subject: |
Re: CXL Namespaces of ACPI disappearing in Qemu demo |
Date: |
Tue, 5 Sep 2023 15:34:38 +0100 |
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