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 <pbonz...@redhat.com> > Cc: "Radim Krčmář" <rkrc...@redhat.com> > Cc: Jonathan Corbet <cor...@lwn.net> > Cc: Thomas Gleixner <t...@linutronix.de> > Cc: Ingo Molnar <mi...@redhat.com> > Cc: "H. Peter Anvin" <h...@zytor.com> > Cc: x...@kernel.org > Cc: Peter Zijlstra <pet...@infradead.org> > Cc: Waiman Long <long...@redhat.com> > Cc: k...@vger.kernel.org > Cc: linux-...@vger.kernel.org > Cc: linux-kernel@vger.kernel.org > Cc: Jan H. Schoenherr <jscho...@amazon.de> > Cc: Anthony Liguori <aligu...@amazon.com> > Suggested-by: Matt Wilson <m...@amazon.com> > Signed-off-by: Eduardo Valentin <edu...@amazon.com> > --- > 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? Thanks.
