Re: [PATCH 1/1] locking/qspinlock/x86: Avoid test-and-set when PV_DEDICATED is set

[Radim KrÄmÃÅ]

2017-10-23 17:44-0700, Eduardo Valentin:
> Currently, the existing qspinlock implementation will fallback to
> test-and-set if the hypervisor has not set the PV_UNHALT flag.

Where have you detected the main source of overhead with pinned VCPUs?
Makes me wonder if we couldn't improve general PV_UNHALT,

thanks.

> This patch gives the opportunity to guest kernels to select
> between test-and-set and the regular queueu fair lock implementation
> based on the PV_DEDICATED KVM feature flag. When the PV_DEDICATED
> flag is not set, the code will still fall back to test-and-set,
> but when the PV_DEDICATED flag is set, the code will use
> the regular queue spinlock implementation.

Some flag makes sense and we do want to make sure that userspaces don't
enable it in pass-through-cpuid mode.