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Date: Mon, 20 Apr 2020 11:47:31 -0700
From: Jon Cargille <jcargill@...gle.com>
To: Vitaly Kuznetsov <vkuznets@...hat.com>
Cc: David Matlack <dmatlack@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Sean Christopherson <sean.j.christopherson@...el.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm: add capability for halt polling
On Sun, Apr 19, 2020 at 1:46 PM Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
>
> Jon Cargille <jcargill@...gle.com> writes:
>
> > From: David Matlack <dmatlack@...gle.com>
> >
> > KVM_CAP_HALT_POLL is a per-VM capability that lets userspace
> > control the halt-polling time, allowing halt-polling to be tuned or
> > disabled on particular VMs.
> >
> > With dynamic halt-polling, a VM's VCPUs can poll from anywhere from
> > [0, halt_poll_ns] on each halt. KVM_CAP_HALT_POLL sets the
> > upper limit on the poll time.
>
> Out of pure curiosity, why is this a per-VM and not a per-VCPU property?
Great question, Vitaly. We actually implemented this as a per-VCPU property
initially; however, our user-space implementation was only using it to apply
the same value to all VCPUs, so we later simplified it on the advice of
Jim Mattson. If there is a consensus for this to go in as per-VCPU rather
than per-VM, I'm happy to submit that way instead. The per-VM version did
end up looking simpler, IMO.
>
> >
> > Signed-off-by: David Matlack <dmatlack@...gle.com>
> > Signed-off-by: Jon Cargille <jcargill@...gle.com>
> > Reviewed-by: Jim Mattson <jmattson@...gle.com>
> > ---
> > Documentation/virt/kvm/api.rst | 17 +++++++++++++++++
> > include/linux/kvm_host.h | 1 +
> > include/uapi/linux/kvm.h | 1 +
> > virt/kvm/kvm_main.c | 19 +++++++++++++++----
> > 4 files changed, 34 insertions(+), 4 deletions(-)
> >
> > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> > index efbbe570aa9b7b..d871dacb984e98 100644
> > --- a/Documentation/virt/kvm/api.rst
> > +++ b/Documentation/virt/kvm/api.rst
> > @@ -5802,6 +5802,23 @@ If present, this capability can be enabled for a VM, meaning that KVM
> > will allow the transition to secure guest mode. Otherwise KVM will
> > veto the transition.
> >
> > +7.20 KVM_CAP_HALT_POLL
> > +----------------------
> > +
> > +:Architectures: all
> > +:Target: VM
> > +:Parameters: args[0] is the maximum poll time in nanoseconds
> > +:Returns: 0 on success; -1 on error
> > +
> > +This capability overrides the kvm module parameter halt_poll_ns for the
> > +target VM.
> > +
> > +VCPU polling allows a VCPU to poll for wakeup events instead of immediately
> > +scheduling during guest halts. The maximum time a VCPU can spend polling is
> > +controlled by the kvm module parameter halt_poll_ns. This capability allows
> > +the maximum halt time to specified on a per-VM basis, effectively overriding
> > +the module parameter for the target VM.
> > +
> > 8. Other capabilities.
> > ======================
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 6d58beb65454f7..922b24ce5e7297 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -503,6 +503,7 @@ struct kvm {
> > struct srcu_struct srcu;
> > struct srcu_struct irq_srcu;
> > pid_t userspace_pid;
> > + unsigned int max_halt_poll_ns;
> > };
> >
> > #define kvm_err(fmt, ...) \
> > diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> > index 428c7dde6b4b37..ac9eba0289d1b6 100644
> > --- a/include/uapi/linux/kvm.h
> > +++ b/include/uapi/linux/kvm.h
> > @@ -1017,6 +1017,7 @@ struct kvm_ppc_resize_hpt {
> > #define KVM_CAP_S390_VCPU_RESETS 179
> > #define KVM_CAP_S390_PROTECTED 180
> > #define KVM_CAP_PPC_SECURE_GUEST 181
> > +#define KVM_CAP_HALT_POLL 182
> >
> > #ifdef KVM_CAP_IRQ_ROUTING
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 74bdb7bf32952e..ec038a9e60a275 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -710,6 +710,8 @@ static struct kvm *kvm_create_vm(unsigned long type)
> > 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;
> > @@ -2716,15 +2718,16 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
> > if (!kvm_arch_no_poll(vcpu)) {
> > if (!vcpu_valid_wakeup(vcpu)) {
> > shrink_halt_poll_ns(vcpu);
> > - } else if (halt_poll_ns) {
> > + } else if (vcpu->kvm->max_halt_poll_ns) {
> > if (block_ns <= vcpu->halt_poll_ns)
> > ;
> > /* we had a long block, shrink polling */
> > - else if (vcpu->halt_poll_ns && block_ns > halt_poll_ns)
> > + else if (vcpu->halt_poll_ns &&
> > + block_ns > vcpu->kvm->max_halt_poll_ns)
> > shrink_halt_poll_ns(vcpu);
> > /* we had a short halt and our poll time is too small */
> > - else if (vcpu->halt_poll_ns < halt_poll_ns &&
> > - block_ns < halt_poll_ns)
> > + else if (vcpu->halt_poll_ns < vcpu->kvm->max_halt_poll_ns &&
> > + block_ns < vcpu->kvm->max_halt_poll_ns)
> > grow_halt_poll_ns(vcpu);
> > } else {
> > vcpu->halt_poll_ns = 0;
> > @@ -3516,6 +3519,7 @@ static long kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
> > case KVM_CAP_IOEVENTFD_ANY_LENGTH:
> > case KVM_CAP_CHECK_EXTENSION_VM:
> > case KVM_CAP_ENABLE_CAP_VM:
> > + case KVM_CAP_HALT_POLL:
> > return 1;
> > #ifdef CONFIG_KVM_MMIO
> > case KVM_CAP_COALESCED_MMIO:
> > @@ -3566,6 +3570,13 @@ static int kvm_vm_ioctl_enable_cap_generic(struct kvm *kvm,
> > return 0;
> > }
> > #endif
> > + case KVM_CAP_HALT_POLL: {
> > + if (cap->flags || cap->args[0] != (unsigned int)cap->args[0])
> > + return -EINVAL;
> > +
> > + kvm->max_halt_poll_ns = cap->args[0];
>
> Is it safe to allow any value from userspace here or would it maybe make
> sense to only allow [0, global halt_poll_ns]?
I believe that any value is safe; a very large value effectively disables
halt-polling, which is equivalent to setting a value of zero to explicitly
disable it, which is legal.
>
>
> > + return 0;
> > + }
> > default:
> > return kvm_vm_ioctl_enable_cap(kvm, cap);
> > }
>
> --
> Vitaly
>
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