Re: [PATCH 4/9] KVM: arm64: Support stolen time reporting via shared structure

Marc Zyngier
2019-08-03T18:13:30Z

On Sat, 3 Aug 2019 18:58:17 +0100
Marc Zyngier <maz@kernel.org> wrote:

> On Fri, 2 Aug 2019 15:50:12 +0100
> Steven Price <steven.price@arm.com> wrote:
>
> > Implement the service call for configuring a shared structre between a
> > VCPU and the hypervisor in which the hypervisor can write the time
> > stolen from the VCPU's execution time by other tasks on the host.
> >
> > The hypervisor allocates memory which is placed at an IPA chosen by user
> > space. The hypervisor then uses WRITE_ONCE() to update the shared
> > structre ensuring single copy atomicity of the 64-bit unsigned value
> > that reports stolen time in nanoseconds.
> >
> > Whenever stolen time is enabled by the guest, the stolen time counter is
> > reset.
> >
> > The stolen time itself is retrieved from the sched_info structure
> > maintained by the Linux scheduler code. We enable SCHEDSTATS when
> > selecting KVM Kconfig to ensure this value is meaningful.
> >
> > Signed-off-by: Steven Price <steven.price@arm.com>
> > ---
> > arch/arm64/include/asm/kvm_host.h | 13 +++++-
> > arch/arm64/kvm/Kconfig | 1 +
> > include/kvm/arm_hypercalls.h | 1 +
> > include/linux/kvm_types.h | 2 +
> > virt/kvm/arm/arm.c | 18 ++++++++
> > virt/kvm/arm/hypercalls.c | 70 +++++++++++++++++++++++++++++++
> > 6 files changed, 104 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index f656169db8c3..78f270190d43 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -44,6 +44,7 @@
> > KVM_ARCH_REQ_FLAGS(0, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
> > #define KVM_REQ_IRQ_PENDING KVM_ARCH_REQ(1)
> > #define KVM_REQ_VCPU_RESET KVM_ARCH_REQ(2)
> > +#define KVM_REQ_RECORD_STEAL KVM_ARCH_REQ(3)
> >
> > DECLARE_STATIC_KEY_FALSE(userspace_irqchip_in_use);
> >
> > @@ -83,6 +84,11 @@ struct kvm_arch {
> >
> > /* Mandated version of PSCI */
> > u32 psci_version;
> > +
> > + struct kvm_arch_pvtime {
> > + void *st;
> > + gpa_t st_base;
> > + } pvtime;
> > };
> >
> > #define KVM_NR_MEM_OBJS 40
> > @@ -338,8 +344,13 @@ struct kvm_vcpu_arch {
> > /* True when deferrable sysregs are loaded on the physical CPU,
> > * see kvm_vcpu_load_sysregs and kvm_vcpu_put_sysregs. */
> > bool sysregs_loaded_on_cpu;
> > -};
> >
> > + /* Guest PV state */
> > + struct {
> > + u64 steal;
> > + u64 last_steal;
> > + } steal;
> > +};
> > /* Pointer to the vcpu's SVE FFR for sve_{save,load}_state() */
> > #define vcpu_sve_pffr(vcpu) ((void *)((char *)((vcpu)->arch.sve_state) + \
> > sve_ffr_offset((vcpu)->arch.sve_max_vl)))
> > diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> > index a67121d419a2..d8b88e40d223 100644
> > --- a/arch/arm64/kvm/Kconfig
> > +++ b/arch/arm64/kvm/Kconfig
> > @@ -39,6 +39,7 @@ config KVM
> > select IRQ_BYPASS_MANAGER
> > select HAVE_KVM_IRQ_BYPASS
> > select HAVE_KVM_VCPU_RUN_PID_CHANGE
> > + select SCHEDSTATS
> > ---help---
> > Support hosting virtualized guest machines.
> > We don't support KVM with 16K page tables yet, due to the multiple
> > diff --git a/include/kvm/arm_hypercalls.h b/include/kvm/arm_hypercalls.h
> > index 35a5abcc4ca3..9f0710ab4292 100644
> > --- a/include/kvm/arm_hypercalls.h
> > +++ b/include/kvm/arm_hypercalls.h
> > @@ -7,6 +7,7 @@
> > #include <asm/kvm_emulate.h>
> >
> > int kvm_hvc_call_handler(struct kvm_vcpu *vcpu);
> > +int kvm_update_stolen_time(struct kvm_vcpu *vcpu);
> >
> > static inline u32 smccc_get_function(struct kvm_vcpu *vcpu)
> > {
> > diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> > index bde5374ae021..1c88e69db3d9 100644
> > --- a/include/linux/kvm_types.h
> > +++ b/include/linux/kvm_types.h
> > @@ -35,6 +35,8 @@ typedef unsigned long gva_t;
> > typedef u64 gpa_t;
> > typedef u64 gfn_t;
> >
> > +#define GPA_INVALID (~(gpa_t)0)
> > +
> > typedef unsigned long hva_t;
> > typedef u64 hpa_t;
> > typedef u64 hfn_t;
> > diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> > index f645c0fbf7ec..ebd963d2580b 100644
> > --- a/virt/kvm/arm/arm.c
> > +++ b/virt/kvm/arm/arm.c
> > @@ -40,6 +40,10 @@
> > #include <asm/kvm_coproc.h>
> > #include <asm/sections.h>
> >
> > +#include <kvm/arm_hypercalls.h>
> > +#include <kvm/arm_pmu.h>
> > +#include <kvm/arm_psci.h>
> > +
> > #ifdef REQUIRES_VIRT
> > __asm__(".arch_extension virt");
> > #endif
> > @@ -135,6 +139,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> > kvm->arch.max_vcpus = vgic_present ?
> > kvm_vgic_get_max_vcpus() : KVM_MAX_VCPUS;
> >
> > + kvm->arch.pvtime.st_base = GPA_INVALID;
> > return ret;
> > out_free_stage2_pgd:
> > kvm_free_stage2_pgd(kvm);
> > @@ -371,6 +376,7 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> > kvm_vcpu_load_sysregs(vcpu);
> > kvm_arch_vcpu_load_fp(vcpu);
> > kvm_vcpu_pmu_restore_guest(vcpu);
> > + kvm_make_request(KVM_REQ_RECORD_STEAL, vcpu);
> >
> > if (single_task_running())
> > vcpu_clear_wfe_traps(vcpu);
> > @@ -617,6 +623,15 @@ static void vcpu_req_sleep(struct kvm_vcpu *vcpu)
> > smp_rmb();
> > }
> >
> > +static void vcpu_req_record_steal(struct kvm_vcpu *vcpu)
> > +{
> > + int idx;
> > +
> > + idx = srcu_read_lock(&vcpu->kvm->srcu);
> > + kvm_update_stolen_time(vcpu);
> > + srcu_read_unlock(&vcpu->kvm->srcu, idx);
> > +}
> > +
> > static int kvm_vcpu_initialized(struct kvm_vcpu *vcpu)
> > {
> > return vcpu->arch.target >= 0;
> > @@ -636,6 +651,9 @@ static void check_vcpu_requests(struct kvm_vcpu *vcpu)
> > * that a VCPU sees new virtual interrupts.
> > */
> > kvm_check_request(KVM_REQ_IRQ_PENDING, vcpu);
> > +
> > + if (kvm_check_request(KVM_REQ_RECORD_STEAL, vcpu))
> > + vcpu_req_record_steal(vcpu);
>
> Something troubles me. Here, you've set the request on load. But you
> can be preempted at any time (preemption gets disabled just after).
>
> I have the feeling that should you get preempted right here, you'll
> end-up having accumulated the wrong amount of steal time, as the
> request put via load when you'll get scheduled back in will only get
> processed after a full round of entry/exit/entry, which doesn't look
> great.

Ah, no. We're saved by the check for pending requests right before we
jump in the guest, causing an early exit and the whole shebang to be
restarted.

M.
--
Without deviation from the norm, progress is not possible.