On Fri, Nov 03, 2017 at 11:09:53AM +0100, Paolo Bonzini wrote: > 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? Sounds reasonable, and it is almost what the patch does. But to achieve the above table, we need in include the following chunk: diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c index 8bb9594..dacd7cf 100644 --- a/arch/x86/kernel/kvm.c +++ b/arch/x86/kernel/kvm.c @@ -642,6 +642,8 @@ void __init kvm_spinlock_init(void) { if (!kvm_para_available()) return; + if (kvm_para_has_feature(KVM_FEATURE_PV_DEDICATED)) + return; /* Does host kernel support KVM_FEATURE_PV_UNHALT? */ if (!kvm_para_has_feature(KVM_FEATURE_PV_UNHALT)) return; Now we get: PV_DEDICATED PV_UNHALT IMPLEMENTATION 1 X qspinlock 0 1 pvspinlock 0 0 tas Do you still think PV_DEDICATED goes as a "configuration hint", given that it would take precedence on a feature (PV_UNHALT)? Or do we keep everything as features (two bits to represent selection of three features)? BR, > > Paolo -- All the best, Eduardo Valentin
