On 02/11/2017 19:43, Eduardo Valentin wrote: > On Thu, Nov 02, 2017 at 07:24:16PM +0100, Paolo Bonzini wrote: >> On 02/11/2017 19:08, Eduardo Valentin wrote: >>> On Thu, Nov 02, 2017 at 06:56:46PM +0100, Paolo Bonzini wrote: >>>> On 02/11/2017 18:45, Eduardo Valentin wrote: >>>>> Currently, the existing qspinlock implementation will fallback to >>>>> test-and-set if the hypervisor has not set the PV_UNHALT flag. >>>>> >>>>> 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. >>>> >>>> Have you seen Waiman's series that lets you specify this on the guest >>>> command line instead? Would this be acceptable for your use case? >>> >>> No, can you please share a link to it? is it already merged to tip/master? >> >> [PATCH-tip v2 0/2] x86/paravirt: Enable users to choose PV lock type >> https://lkml.org/lkml/2017/11/1/655 >> >>>> (In other words, is there a difference for you between making the host >>>> vs. guest administrator toggle the feature? "@amazon.com" means you are >>>> the host admin, how would you use it?) >>> >>> The way I think of this is this is a flag set by host side so the >>> guest adapts accordingly. >>> >>> If the admin in guest side wants to ignore what the host is >>> flagging, that is a different story. >> >> Okay, this makes sense. But perhaps it should be a separate CPUID leaf, >> such as "configuration hints", rather than properly a feature. > > Oh OK, you don't think this starts to deviate from the feature concept. > But would the PV_UNHALT also go to "configuration hints" bucket? PV_UNHALT says whether the pvqspinlock API is available, PV_DEDICATED says whether it should be used. > Another way to see this is we have three locking feature options to select from, > so we need at least two bits here. PV_DEDICATED = 1, PV_UNHALT = anything: default is qspinlock PV_DEDICATED = 0, PV_UNHALT = 1: default is pvqspinlock PV_DEDICATED = 0, PV_UNHALT = 0: default is tas What do you think? Paolo
