Re: [PATCH 0/7] x86/resctrl : Support L3 Smart Data Cache Injection Allocation Enforcement (SDCIAE)

Reinette Chatre
2024-09-13T20:44:24Z

Hi Babu,

On 8/16/24 9:16 AM, Babu Moger wrote:
>
> This series adds the support for L3 Smart Data Cache Injection Allocation
> Enforcement (SDCIAE) to resctrl infrastructure.
>
> Upcoming AMD hardware implements Smart Data Cache Injection (SDCI).
> Smart Data Cache Injection (SDCI) is a mechanism that enables direct
> insertion of data from I/O devices into the L3 cache. By directly caching
> data from I/O devices rather than first storing the I/O data in DRAM, SDCI
> reduces demands on DRAM bandwidth and reduces latency to the processor
> consuming the I/O data. The SDCIAE (SDCI Allocation Enforcement) PQE
> feature allows system software to limit the portion of the L3 cache used
> for SDCI.
>

This series introduces new user interface. Could you please describe the
new user interface in the cover letter and how users are expected to interact
with this interface to be able to use this new feature? Please also describe
the impact on existing resctrl interfaces related to cache allocation from
I/O hardware, for example "shareable_bits", "bit_usage", etc. These existing
interfaces are used to communicate to user space how portions of cache are
used by I/O hardware but I cannot tell from this series how this work builds on
this.

How does this feature work with the existing "L3 Cache Allocation Sharing Mask"
that is enumerated as part of CAT feature?

> The feature details are documented in the APM listed below [1].
> [1] AMD64 Architecture Programmer's Manual Volume 2: System Programming
> Publication # 24593 Revision 3.41 section 19.4.7 L3 Smart Data Cache
> Injection Allocation Enforcement (SDCIAE)
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537
>
> The feature requires linux support of TPH (TLP Processing Hints).
> The support is ongoing and patches are currently under review.
> https://lore.kernel.org/lkml/20240717205511.2541693-2-wei.huang2@amd.com/

Please note that the cover letter [1] of that series mentions "Cache Injection
allows PCIe endpoints to inject I/O Coherent DMA writes directly into an L2 ..."
while this series implements and refers to L3 only.

>
> The patches are based on top of commit
> ad1b4b6ed19f (tip/master) Merge branch into tip/master: 'x86/timers'
>

Reinette

[1] https://lore.kernel.org/lkml/20240717205511.2541693-1-wei.huang2@amd.com/