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

    Am I getting it right?

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

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