[RFC PATCH] KVM: arm64: vgic-v3: Cache ICC_CTLR_EL1 and allow lockless read when ready

[Marc Zyngier]

On Wed, 08 Oct 2025 21:19:55 +0100,
salil.mehta at opnsrc.net wrote:
> 
> From: Salil Mehta <salil.mehta at huawei.com>
> 
> [A rough illustration of the problem and the probable solution]
> 
> Userspace reads of ICC_CTLR_EL1 via KVM device attributes currently takes a slow
> path that may acquire all vCPU locks. Under workloads that exercise userspace
> PSCI CPU_ON flows or frequent vCPU resets, this can cause vCPU lock contention
> in KVM and, in the worst cases, -EBUSY returns to userspace.
> 
> When PSCI CPU_ON and CPU_OFF calls are handled entirely in KVM, these operations
> are executed under KVM vCPU locks in the host kernel (EL1) and appear atomic to
> other vCPU threads. In this context, system register accesses are serialized
> under KVM vCPU locks, ensuring atomicity with respect to other vCPUs. After
> SMCCC filtering was introduced, PSCI CPU_ON and CPU_OFF calls can now exit to
> userspace (QEMU). During the handling of PSCI CPU_ON call in userspace, a
> cpu_reset() is exerted which reads ICC_CTLR_EL1 through KVM device attribute
> IOCTLs. To avoid transient inconsistency and -EBUSY errors, QEMU is forced to
> pause all vCPUs before issuing these IOCTLs.

I'm going to repeat in public what I already said in private.

Why does QEMU need to know this? I don't see how this is related to
PSCI, and outside of save/restore, there is no reason why QEMU should
poke at this. If QEMU needs fixing, please fix QEMU.

Honestly, I don't see why the kernel should even care about this, and
I have no intention of adopting anything of the sort for something
that has all the hallmarks of a userspace bug.

	M.

-- 
Without deviation from the norm, progress is not possible.