Hello, On Wed, Nov 08, 2017 at 06:36:52PM +0100, Radim Krčmář wrote: > 2017-11-06 12:26-0800, Eduardo Valentin: > > 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. > > > > With this patch, when in autoselect mode, the guest will > > use the default spinlock implementation based on host feature > > flags as follows: > > > > 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 > > > > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> > > Cc: "Radim Krčmář" <rkrcmar@xxxxxxxxxx> > > Cc: Jonathan Corbet <corbet@xxxxxxx> > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> > > Cc: x86@xxxxxxxxxx > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > > Cc: Waiman Long <longman@xxxxxxxxxx> > > Cc: kvm@xxxxxxxxxxxxxxx > > Cc: linux-doc@xxxxxxxxxxxxxxx > > Cc: linux-kernel@xxxxxxxxxxxxxxx > > Cc: Jan H. Schoenherr <jschoenh@xxxxxxxxx> > > Cc: Anthony Liguori <aliguori@xxxxxxxxxx> > > Suggested-by: Matt Wilson <msw@xxxxxxxxxx> > > Signed-off-by: Eduardo Valentin <eduval@xxxxxxxxxx> > > --- > > V3: > > - When PV_DEDICATED is set (1), qspinlock is selected, > > regardless of the value of PV_UNHAULT. Suggested by Paolo Bonzini. > > - Refreshed on top of tip/master. > > V2: > > - rebase on top of tip/master > > > > Documentation/virtual/kvm/cpuid.txt | 6 ++++++ > > arch/x86/include/asm/qspinlock.h | 4 ++++ > > arch/x86/include/uapi/asm/kvm_para.h | 1 + > > arch/x86/kernel/kvm.c | 2 ++ > > 4 files changed, 13 insertions(+) > > > > diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt > > index 3c65feb..117066a 100644 > > --- a/Documentation/virtual/kvm/cpuid.txt > > +++ b/Documentation/virtual/kvm/cpuid.txt > > @@ -54,6 +54,12 @@ KVM_FEATURE_PV_UNHALT || 7 || guest checks this feature bit > > || || before enabling paravirtualized > > || || spinlock support. > > ------------------------------------------------------------------------------ > > +KVM_FEATURE_PV_DEDICATED || 8 || guest checks this feature bit > > + || || to determine if they run on > > + || || dedicated vCPUs, allowing opti- > > + || || mizations such as usage of > > + || || qspinlocks. > > +------------------------------------------------------------------------------ > > KVM_FEATURE_CLOCKSOURCE_STABLE_BIT || 24 || host will warn if no guest-side > > || || per-cpu warps are expected in > > || || kvmclock. > > diff --git a/arch/x86/include/asm/qspinlock.h b/arch/x86/include/asm/qspinlock.h > > index 5e16b5d..de42694 100644 > > --- a/arch/x86/include/asm/qspinlock.h > > +++ b/arch/x86/include/asm/qspinlock.h > > @@ -3,6 +3,8 @@ > > #define _ASM_X86_QSPINLOCK_H > > > > #include <linux/jump_label.h> > > +#include <linux/kvm_para.h> > > + > > #include <asm/cpufeature.h> > > #include <asm-generic/qspinlock_types.h> > > #include <asm/paravirt.h> > > @@ -58,6 +60,8 @@ static inline bool virt_spin_lock(struct qspinlock *lock) > > if (!static_branch_likely(&virt_spin_lock_key)) > > return false; > > > > + if (kvm_para_has_feature(KVM_FEATURE_PV_DEDICATED)) > > + return false; > > Hm, every spinlock slowpath calls cpuid, which causes a VM exit, so I > wouldn't expect it to be faster than the existing implementations. > (Using the static key would be better.) > > How does this patch perform compared to user-forced qspinlock and hybrid > pvqspinlock? This patch should have same effect as user-forced qspinlock. However, the key aspect here is this patch gives a way for the host to instruct the guest to use qspinlock. Even with Longman's patch which allows guest to select the spinlock implementation, there should still be the auto-select mode. In such mode, PV_DEDICATED should allow the host to get the guest to use qspinlock, without, the guest will fallback to tas when PV_UNHALT == 0. > > Thanks. > -- All the best, Eduardo Valentin
