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Message-ID: <20170130144550.GA16459@cbox>
Date: Mon, 30 Jan 2017 15:45:50 +0100
From: Christoffer Dall <christoffer.dall@...aro.org>
To: Marc Zyngier <marc.zyngier@....com>
Cc: Jintack Lim <jintack@...columbia.edu>, pbonzini@...hat.com,
rkrcmar@...hat.com, linux@...linux.org.uk, catalin.marinas@....com,
will.deacon@....com, andre.przywara@....com, kvm@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
linux-kernel@...r.kernel.org
Subject: Re: [RFC v2 02/10] KVM: arm/arm64: Move cntvoff to each timer context
On Sun, Jan 29, 2017 at 11:54:05AM +0000, Marc Zyngier wrote:
> On Fri, Jan 27 2017 at 01:04:52 AM, Jintack Lim <jintack@...columbia.edu> wrote:
> > Make cntvoff per each timer context. This is helpful to abstract kvm
> > timer functions to work with timer context without considering timer
> > types (e.g. physical timer or virtual timer).
> >
> > This also would pave the way for ever doing adjustments of the cntvoff
> > on a per-CPU basis if that should ever make sense.
> >
> > Signed-off-by: Jintack Lim <jintack@...columbia.edu>
> > ---
> > arch/arm/include/asm/kvm_host.h | 6 +++---
> > arch/arm64/include/asm/kvm_host.h | 4 ++--
> > include/kvm/arm_arch_timer.h | 8 +++-----
> > virt/kvm/arm/arch_timer.c | 26 ++++++++++++++++++++------
> > virt/kvm/arm/hyp/timer-sr.c | 3 +--
> > 5 files changed, 29 insertions(+), 18 deletions(-)
> >
> > diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> > index d5423ab..f5456a9 100644
> > --- a/arch/arm/include/asm/kvm_host.h
> > +++ b/arch/arm/include/asm/kvm_host.h
> > @@ -60,9 +60,6 @@ struct kvm_arch {
> > /* The last vcpu id that ran on each physical CPU */
> > int __percpu *last_vcpu_ran;
> >
> > - /* Timer */
> > - struct arch_timer_kvm timer;
> > -
> > /*
> > * Anything that is not used directly from assembly code goes
> > * here.
> > @@ -75,6 +72,9 @@ struct kvm_arch {
> > /* Stage-2 page table */
> > pgd_t *pgd;
> >
> > + /* A lock to synchronize cntvoff among all vtimer context of vcpus */
> > + spinlock_t cntvoff_lock;
>
> Is there any condition where we need this to be a spinlock? I would have
> thought that a mutex should have been enough, as this should only be
> updated on migration or initialization. Not that it matters much in this
> case, but I wondered if there is something I'm missing.
>
I would think the critical section is small enough that a spinlock makes
sense, but what I don't think we need is to add the additional lock.
I think just taking the kvm->lock should be sufficient, which happens to
be a mutex, and while that may be a bit slower to take than the
spinlock, it's not in the critical path so let's just keep things
simple.
Perhaps this what Marc also meant.
> > +
> > /* Interrupt controller */
> > struct vgic_dist vgic;
> > int max_vcpus;
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index e505038..23749a8 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -71,8 +71,8 @@ struct kvm_arch {
> > /* Interrupt controller */
> > struct vgic_dist vgic;
> >
> > - /* Timer */
> > - struct arch_timer_kvm timer;
> > + /* A lock to synchronize cntvoff among all vtimer context of vcpus */
> > + spinlock_t cntvoff_lock;
> > };
> >
> > #define KVM_NR_MEM_OBJS 40
> > diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
> > index daad3c1..1b9c988 100644
> > --- a/include/kvm/arm_arch_timer.h
> > +++ b/include/kvm/arm_arch_timer.h
> > @@ -23,11 +23,6 @@
> > #include <linux/hrtimer.h>
> > #include <linux/workqueue.h>
> >
> > -struct arch_timer_kvm {
> > - /* Virtual offset */
> > - u64 cntvoff;
> > -};
> > -
> > struct arch_timer_context {
> > /* Registers: control register, timer value */
> > u32 cnt_ctl;
> > @@ -38,6 +33,9 @@ struct arch_timer_context {
> >
> > /* Active IRQ state caching */
> > bool active_cleared_last;
> > +
> > + /* Virtual offset */
> > + u64 cntvoff;
> > };
> >
> > struct arch_timer_cpu {
> > diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> > index 6740efa..fa4c042 100644
> > --- a/virt/kvm/arm/arch_timer.c
> > +++ b/virt/kvm/arm/arch_timer.c
> > @@ -101,9 +101,10 @@ static void kvm_timer_inject_irq_work(struct work_struct *work)
> > static u64 kvm_timer_compute_delta(struct kvm_vcpu *vcpu)
> > {
> > u64 cval, now;
> > + struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
> >
> > - cval = vcpu_vtimer(vcpu)->cnt_cval;
> > - now = kvm_phys_timer_read() - vcpu->kvm->arch.timer.cntvoff;
> > + cval = vtimer->cnt_cval;
> > + now = kvm_phys_timer_read() - vtimer->cntvoff;
> >
> > if (now < cval) {
> > u64 ns;
> > @@ -159,7 +160,7 @@ bool kvm_timer_should_fire(struct kvm_vcpu *vcpu)
> > return false;
> >
> > cval = vtimer->cnt_cval;
> > - now = kvm_phys_timer_read() - vcpu->kvm->arch.timer.cntvoff;
> > + now = kvm_phys_timer_read() - vtimer->cntvoff;
> >
> > return cval <= now;
> > }
> > @@ -353,10 +354,23 @@ int kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu,
> > return 0;
> > }
> >
> > +/* Make the updates of cntvoff for all vtimer contexts atomic */
> > +static void update_vtimer_cntvoff(struct kvm_vcpu *vcpu, u64 cntvoff)
>
> Arguably, this acts on the VM itself and not a single vcpu. maybe you
> should consider passing the struct kvm pointer to reflect this.
>
> > +{
> > + int i;
> > +
> > + spin_lock(&vcpu->kvm->arch.cntvoff_lock);
> > + kvm_for_each_vcpu(i, vcpu, vcpu->kvm)
> > + vcpu_vtimer(vcpu)->cntvoff = cntvoff;
> > + spin_unlock(&vcpu->kvm->arch.cntvoff_lock);
> > +}
> > +
> > void kvm_timer_vcpu_init(struct kvm_vcpu *vcpu)
> > {
> > struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
> >
> > + update_vtimer_cntvoff(vcpu, kvm_phys_timer_read());
>
> Maybe a comment indicating that we recompute CNTVOFF for all vcpus would
> be welcome (this is not a change in semantics, but it was never obvious
> in the existing code).
>
> > +
> > INIT_WORK(&timer->expired, kvm_timer_inject_irq_work);
> > hrtimer_init(&timer->timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
> > timer->timer.function = kvm_timer_expire;
> > @@ -376,7 +390,7 @@ int kvm_arm_timer_set_reg(struct kvm_vcpu *vcpu, u64 regid, u64 value)
> > vtimer->cnt_ctl = value;
> > break;
> > case KVM_REG_ARM_TIMER_CNT:
> > - vcpu->kvm->arch.timer.cntvoff = kvm_phys_timer_read() - value;
> > + update_vtimer_cntvoff(vcpu, kvm_phys_timer_read() - value);
> > break;
> > case KVM_REG_ARM_TIMER_CVAL:
> > vtimer->cnt_cval = value;
> > @@ -397,7 +411,7 @@ u64 kvm_arm_timer_get_reg(struct kvm_vcpu *vcpu, u64 regid)
> > case KVM_REG_ARM_TIMER_CTL:
> > return vtimer->cnt_ctl;
> > case KVM_REG_ARM_TIMER_CNT:
> > - return kvm_phys_timer_read() - vcpu->kvm->arch.timer.cntvoff;
> > + return kvm_phys_timer_read() - vtimer->cntvoff;
> > case KVM_REG_ARM_TIMER_CVAL:
> > return vtimer->cnt_cval;
> > }
> > @@ -511,7 +525,7 @@ int kvm_timer_enable(struct kvm_vcpu *vcpu)
> >
> > void kvm_timer_init(struct kvm *kvm)
> > {
> > - kvm->arch.timer.cntvoff = kvm_phys_timer_read();
> > + spin_lock_init(&kvm->arch.cntvoff_lock);
> > }
> >
> > /*
> > diff --git a/virt/kvm/arm/hyp/timer-sr.c b/virt/kvm/arm/hyp/timer-sr.c
> > index 0cf0895..4734915 100644
> > --- a/virt/kvm/arm/hyp/timer-sr.c
> > +++ b/virt/kvm/arm/hyp/timer-sr.c
> > @@ -53,7 +53,6 @@ void __hyp_text __timer_save_state(struct kvm_vcpu *vcpu)
> >
> > void __hyp_text __timer_restore_state(struct kvm_vcpu *vcpu)
> > {
> > - struct kvm *kvm = kern_hyp_va(vcpu->kvm);
> > struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
> > struct arch_timer_context *vtimer = vcpu_vtimer(vcpu);
> > u64 val;
> > @@ -71,7 +70,7 @@ void __hyp_text __timer_restore_state(struct kvm_vcpu *vcpu)
> > }
> >
> > if (timer->enabled) {
> > - write_sysreg(kvm->arch.timer.cntvoff, cntvoff_el2);
> > + write_sysreg(vtimer->cntvoff, cntvoff_el2);
> > write_sysreg_el0(vtimer->cnt_cval, cntv_cval);
> > isb();
> > write_sysreg_el0(vtimer->cnt_ctl, cntv_ctl);
>
I agree with the other two comments as well.
Otherwise looks ok.
Thanks,
-Christoffer
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