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Re: [RFC PATCH 00/12] hw/arm/virt: Introduce cpu and cache topology supp

From: Ying Fang
Subject: Re: [RFC PATCH 00/12] hw/arm/virt: Introduce cpu and cache topology support
Date: Fri, 16 Oct 2020 17:40:02 +0800
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0

On 10/15/2020 3:59 PM, Andrew Jones wrote:
On Thu, Oct 15, 2020 at 10:07:16AM +0800, Ying Fang wrote:

On 10/14/2020 2:08 AM, Andrew Jones wrote:
On Tue, Oct 13, 2020 at 12:11:20PM +0000, Zengtao (B) wrote:
Cc valentin

-----Original Message-----
From: Qemu-devel
On Behalf Of Ying Fang
Sent: Thursday, September 17, 2020 11:20 AM
To: qemu-devel@nongnu.org
Cc: peter.maydell@linaro.org; drjones@redhat.com; Zhanghailiang;
Chenzhendong (alex); shannon.zhaosl@gmail.com;
qemu-arm@nongnu.org; alistair.francis@wdc.com; fangying;
Subject: [RFC PATCH 00/12] hw/arm/virt: Introduce cpu and cache
topology support

An accurate cpu topology may help improve the cpu scheduler's
making when dealing with multi-core system. So cpu topology
is helpful to provide guest with the right view. Cpu cache information
also have slight impact on the sched domain, and even userspace
may check the cpu cache information to do some optimizations. Thus
this patch
series is posted to provide cpu and cache topology support for arm.

To make the cpu topology consistent with MPIDR, an vcpu ioctl

For aarch64, the cpu topology don't depends on the MPDIR.
See https://patchwork.kernel.org/patch/11744387/

The topology should not be inferred from the MPIDR Aff fields,

MPIDR is abused by ARM OEM manufactures. It is only used as a
identifer for a specific cpu, not representation of the topology.

Right, which is why I stated topology should not be inferred from

but MPIDR is the CPU identifier. When describing a topology
with ACPI or DT the CPU elements in the topology description
must map to actual CPUs. MPIDR is that mapping link. KVM
currently determines what the MPIDR of a VCPU is. If KVM

KVM currently assigns MPIDR with vcpu->vcpu_id which mapped
into affinity levels. See reset_mpidr in sys_regs.c

I know, but how KVM assigns MPIDRs today is not really important
to KVM userspace. KVM userspace shouldn't depend on a KVM
algorithm, as it could change.

userspace is going to determine the VCPU topology, then it
also needs control over the MPIDR values, otherwise it
becomes quite messy trying to get the mapping right.
If we are going to control MPIDR, shall we assign MPIDR with
vcpu_id or map topology hierarchy into affinity levels or any
other link schema ?

We can assign them to whatever we want, as long as they're
unique and as long as Aff0 is assigned per the GIC requirements,
e.g. GICv3 requires that Aff0 be from 0 to 0xf. Also, when
pinning VCPUs to PCPUs we should ensure that MPIDRs with matching
Aff3,Aff2,Aff1 fields should actually be peers with respect to
the GIC.

Still not clear why vCPU's MPIDR need to match pPCPU's GIC affinity.
Maybe I should read spec for GICv3.

We shouldn't try to encode topology in the MPIDR in any way,
so we might as well simply increment a counter to assign them,
which could possibly be the same as the VCPU ID.

Hmm, then we can leave it as it is.



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