On Tue, 5 Mar 2019 13:36:30 +0100
Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
> On 05.03.2019 12:03, Cornelia Huck wrote:
> > On Tue, 5 Mar 2019 05:30:01 -0500
> > Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
> >
> >> There are cases where halt polling is unwanted. For example when running
> >> KVM on an over committed LPAR we rather want to give back the CPU to
> >> neighbour LPARs instead of polling. Let us provide a callback that
> >> allows architectures to disable polling.
> >>
> >> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> >> ---
> >> include/linux/kvm_host.h | 10 ++++++++++
> >> virt/kvm/Kconfig | 3 +++
> >> virt/kvm/kvm_main.c | 2 +-
> >> 3 files changed, 14 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> >> index c38cc5eb7e73..5d2bbcf0b1de 100644
> >> --- a/include/linux/kvm_host.h
> >> +++ b/include/linux/kvm_host.h
> >> @@ -1283,6 +1283,16 @@ static inline bool vcpu_valid_wakeup(struct kvm_vcpu *vcpu)
> >> }
> >> #endif /* CONFIG_HAVE_KVM_INVALID_WAKEUPS */
> >>
> >> +#ifdef CONFIG_HAVE_KVM_NO_POLL
> >> +/* Callback that tells if we must not poll */
> >> +bool kvm_arch_no_poll(void);
> >> +#else
> >> +static inline bool kvm_arch_no_poll(void)
> >> +{
> >> + return false;
> >> +}
> >> +#endif /* CONFIG_HAVE_KVM_NO_POLL */
> >> +
> >> #ifdef CONFIG_HAVE_KVM_VCPU_ASYNC_IOCTL
> >> long kvm_arch_vcpu_async_ioctl(struct file *filp,
> >> unsigned int ioctl, unsigned long arg);
> >> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
> >> index ea434ddc8499..aad9284c043a 100644
> >> --- a/virt/kvm/Kconfig
> >> +++ b/virt/kvm/Kconfig
> >> @@ -57,3 +57,6 @@ config HAVE_KVM_VCPU_ASYNC_IOCTL
> >>
> >> config HAVE_KVM_VCPU_RUN_PID_CHANGE
> >> bool
> >> +
> >> +config HAVE_KVM_NO_POLL
> >> + bool
> >> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> >> index 076bc38963bf..c76d83532e14 100644
> >> --- a/virt/kvm/kvm_main.c
> >> +++ b/virt/kvm/kvm_main.c
> >> @@ -2248,7 +2248,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> >> u64 block_ns;
> >>
> >> start = cur = ktime_get();
> >> - if (vcpu->halt_poll_ns) {
> >> + if (vcpu->halt_poll_ns && !kvm_arch_no_poll()) {
> >> ktime_t stop = ktime_add_ns(ktime_get(), vcpu->halt_poll_ns);
> >>
> >> ++vcpu->stat.halt_attempted_poll;
> >
> > What this gives us is a way to check whether we want to disable halt
> > polling right now (e.g. because we have a high steal time right now).
> > Both the drawback and the point in favour is that we check this every
> > time we enter kvm_vcpu_block().
> >
> > Are there (known) scenarios where we might want to disable halt polling
> > for some period of time? E.g., when we know via some event that it will
> > be counterproductive for some time or until we get an event that tells
> > us it might make sense again?
>
> For these cases we probably want to have a userspace tool that sets/unsets
> /sys/module/kvm/parameters/halt_poll_ns?
Yes, if userspace has/can get the relevant information, that makes
sense.
