Re: [PATCH v7 RESEND 0/5] i386: Support SMP Cache Topology

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jan 10, 2025 at 10:51:10PM +0800, Zhao Liu wrote:
> Hi folks,
> 
> This is my v7 resend version (updated the commit message of origin
> v7's Patch 1).
> 
> Compared with v6 [1], v7 dropped the "thread" level cache topology
> (cache per thread):
> 
>  - Patch 1 is the new patch to reject "thread" parameter for smp-cache.
>  - Ptach 2 dropped cache per thread support.
>  (Others remain unchanged.)
> 
> There're several reasons:
> 
>  * Currently, neither i386 nor ARM have real hardware support for per-
>    thread cache.
>  * ARM can't support thread level cache in device tree. [2].
> 
> So it is unnecessary to support it at this moment, even though per-
> thread cache might have potential scheduling benefits for VMs without
> CPU affinity.
> 
> In the future, if there is a clear demand for this feature, the correct
> approach would be to add a new control field in MachineClass.smp_props
> and enable it only for the machines that require it.
> 
> 
> This series is based on the master branch at commit aa3a285b5bc5 ("Merge
> tag 'mem-2024-12-21' of https://github.com/davidhildenbrand/qemu into
> staging").

pc things:

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>



> Smp-cache support of ARM side can be found at [3].
> 
> 
> Background
> ==========
> 
> The x86 and ARM (RISCV) need to allow user to configure cache properties
> (current only topology):
>  * For x86, the default cache topology model (of max/host CPU) does not
>    always match the Host's real physical cache topology. Performance can
>    increase when the configured virtual topology is closer to the
>    physical topology than a default topology would be.
>  * For ARM, QEMU can't get the cache topology information from the CPU
>    registers, then user configuration is necessary. Additionally, the
>    cache information is also needed for MPAM emulation (for TCG) to
>    build the right PPTT. (Originally from Jonathan)
> 
> 
> About smp-cache
> ===============
> 
> The API design has been discussed heavily in [4].
> 
> Now, smp-cache is implemented as a array integrated in -machine. Though
> -machine currently can't support JSON format, this is the one of the
> directions of future.
> 
> An example is as follows:
> 
> smp_cache=smp-cache.0.cache=l1i,smp-cache.0.topology=core,smp-cache.1.cache=l1d,smp-cache.1.topology=core,smp-cache.2.cache=l2,smp-cache.2.topology=module,smp-cache.3.cache=l3,smp-cache.3.topology=die
> 
> "cache" specifies the cache that the properties will be applied on. This
> field is the combination of cache level and cache type. Now it supports
> "l1d" (L1 data cache), "l1i" (L1 instruction cache), "l2" (L2 unified
> cache) and "l3" (L3 unified cache).
> 
> "topology" field accepts CPU topology levels including "core", "module",
> "cluster", "die", "socket", "book", "drawer" and a special value
> "default". (Note, now, in v7, smp-cache doesn't support "thread".)
> 
> The "default" is introduced to make it easier for libvirt to set a
> default parameter value without having to care about the specific
> machine (because currently there is no proper way for machine to
> expose supported topology levels and caches).
> 
> If "default" is set, then the cache topology will follow the
> architecture's default cache topology model. If other CPU topology level
> is set, the cache will be shared at corresponding CPU topology level.
> 
> [1]: Patch v6: https://lore.kernel.org/qemu-devel/20241219083237.265419-1-zhao1.liu@intel.com/
> [2]: Gap of cache per thread for ARM: https://lore.kernel.org/qemu-devel/20250110114100.00002296@huawei.com/T/#m50c37fa5d372feac8e607c279cd446da3e22a12c
> [3]: ARM smp-cache: https://lore.kernel.org/qemu-devel/20250102152012.1049-1-alireza.sanaee@huawei.com/
> [4]: API disscussion: https://lore.kernel.org/qemu-devel/8734ndj33j.fsf@pond.sub.org/
> 
> Thanks and Best Regards,
> Zhao
> ---
> Alireza Sanaee (1):
>   i386/cpu: add has_caches flag to check smp_cache configuration
> 
> Zhao Liu (4):
>   hw/core/machine: Reject thread level cache
>   i386/cpu: Support module level cache topology
>   i386/cpu: Update cache topology with machine's configuration
>   i386/pc: Support cache topology in -machine for PC machine
> 
>  hw/core/machine-smp.c |  9 ++++++
>  hw/i386/pc.c          |  4 +++
>  include/hw/boards.h   |  3 ++
>  qemu-options.hx       | 30 +++++++++++++++++-
>  target/i386/cpu.c     | 71 ++++++++++++++++++++++++++++++++++++++++++-
>  5 files changed, 115 insertions(+), 2 deletions(-)
> 
> -- 
> 2.34.1