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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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