On Fri, Nov 18, 2022 at 8:58 AM David Matlack <dmatlack@xxxxxxxxxx> wrote:
>
> On Fri, Nov 18, 2022 at 12:28 AM wangyanan (Y) <wangyanan55@xxxxxxxxxx> wrote:
> >
> > Hi David,
> >
> > On 2022/11/17 8:16, David Matlack wrote:
> > > Obey kvm.halt_poll_ns in VMs not using KVM_CAP_HALT_POLL on every halt,
> > > rather than just sampling the module parameter when the VM is first
> > s/first/firstly
> > > created. This restore the original behavior of kvm.halt_poll_ns for VMs
> > s/restore/restores
> > > that have not opted into KVM_CAP_HALT_POLL.
> > >
> > > Notably, this change restores the ability for admins to disable or
> > > change the maximum halt-polling time system wide for VMs not using
> > > KVM_CAP_HALT_POLL.
> > Should we add more detailed comments about relationship
> > between KVM_CAP_HALT_POLL and kvm.halt_poll_ns in
> > Documentation/virt/kvm/api.rst? Something like:
> > "once KVM_CAP_HALT_POLL is used for a target VM, it will
> > completely ignores any future changes to kvm.halt_poll_ns..."
>
> Yes we should.
>
> I will do some testing on this series today and then resend the series
> as a non-RFC with the Documentation changes.
>
> Thanks for the reviews.
Initial testing looks good but I did not have time to finish due to a
bug in how our VMM is currently using KVM_CAP_HALT_POLL. I will be out
all next week so I'll pick this up the week after.
>
> > > Reported-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
> > > Fixes: acd05785e48c ("kvm: add capability for halt polling")
> > > Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx>
> > > ---
> > > include/linux/kvm_host.h | 1 +
> > > virt/kvm/kvm_main.c | 27 ++++++++++++++++++++++++---
> > > 2 files changed, 25 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > > index e6e66c5e56f2..253ad055b6ad 100644
> > > --- a/include/linux/kvm_host.h
> > > +++ b/include/linux/kvm_host.h
> > > @@ -788,6 +788,7 @@ struct kvm {
> > > struct srcu_struct srcu;
> > > struct srcu_struct irq_srcu;
> > > pid_t userspace_pid;
> > > + bool override_halt_poll_ns;
> > > unsigned int max_halt_poll_ns;
> > > u32 dirty_ring_size;
> > > bool vm_bugged;
> > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > > index 78caf19608eb..7f73ce99bd0e 100644
> > > --- a/virt/kvm/kvm_main.c
> > > +++ b/virt/kvm/kvm_main.c
> > > @@ -1198,8 +1198,6 @@ static struct kvm *kvm_create_vm(unsigned long type, const char *fdname)
> > > goto out_err_no_arch_destroy_vm;
> > > }
> > >
> > > - kvm->max_halt_poll_ns = halt_poll_ns;
> > > -
> > > r = kvm_arch_init_vm(kvm, type);
> > > if (r)
> > > goto out_err_no_arch_destroy_vm;
> > > @@ -3490,7 +3488,20 @@ static inline void update_halt_poll_stats(struct kvm_vcpu *vcpu, ktime_t start,
> > >
> > > static unsigned int kvm_vcpu_max_halt_poll_ns(struct kvm_vcpu *vcpu)
> > > {
> > > - return READ_ONCE(vcpu->kvm->max_halt_poll_ns);
> > > + struct kvm *kvm = vcpu->kvm;
> > > +
> > > + if (kvm->override_halt_poll_ns) {
> > > + /*
> > > + * Ensure kvm->max_halt_poll_ns is not read before
> > > + * kvm->override_halt_poll_ns.
> > > + *
> > > + * Pairs with the smp_wmb() when enabling KVM_CAP_HALT_POLL.
> > > + */
> > > + smp_rmb();
> > > + return READ_ONCE(kvm->max_halt_poll_ns);
> > > + }
> > > +
> > > + return READ_ONCE(halt_poll_ns);
> > > }
> > >
> > > /*
> > > @@ -4600,6 +4611,16 @@ static int kvm_vm_ioctl_enable_cap_generic(struct kvm *kvm,
> > > return -EINVAL;
> > >
> > > kvm->max_halt_poll_ns = cap->args[0];
> > > +
> > > + /*
> > > + * Ensure kvm->override_halt_poll_ns does not become visible
> > > + * before kvm->max_halt_poll_ns.
> > > + *
> > > + * Pairs with the smp_rmb() in kvm_vcpu_max_halt_poll_ns().
> > > + */
> > > + smp_wmb();
> > > + kvm->override_halt_poll_ns = true;
> > > +
> > > return 0;
> > > }
> > > case KVM_CAP_DIRTY_LOG_RING:
> > Looks good to me:
> > Reviewed-by: Yanan Wang <wangyanan55@xxxxxxxxxx>
> >
> > Thanks,
> > Yanan
