[PATCH v3 2/3] KVM: x86: Include host suspended time in steal time.

Suleiman Souhlal
2025-01-07T04:26:06Z

When the host resumes from a suspend, the guest thinks any task
that was running during the suspend ran for a long time, even though
the effective run time was much shorter, which can end up having
negative effects with scheduling. This can be particularly noticeable
if the guest task was RT, as it can end up getting throttled for a
long time.

To mitigate this issue, we include the time that the host was
suspended in steal time, which lets the guest can subtract the
duration from the tasks' runtime.

Note that the case of a suspend happening during a VM migration
might not be accounted.

Change-Id: I18d1d17d4d0d6f4c89b312e427036e052c47e1fa
Signed-off-by: Suleiman Souhlal <suleiman@google.com>
---
arch/x86/include/asm/kvm_host.h | 1 +
arch/x86/kvm/x86.c | 11 ++++++++++-
2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e159e44a6a1b61..01d44d527a7f88 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -897,6 +897,7 @@ struct kvm_vcpu_arch {
u8 preempted;
u64 msr_val;
u64 last_steal;
+ u64 last_suspend_ns;
struct gfn_to_hva_cache cache;
} st;

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c8160baf383851..12439edc36f83a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3650,7 +3650,7 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
struct kvm_steal_time __user *st;
struct kvm_memslots *slots;
gpa_t gpa = vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS;
- u64 steal;
+ u64 steal, suspend_ns;
u32 version;

if (kvm_xen_msr_enabled(vcpu->kvm)) {
@@ -3677,6 +3677,7 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
return;
}

+ suspend_ns = kvm_total_suspend_ns();
st = (struct kvm_steal_time __user *)ghc->hva;
/*
* Doing a TLB flush here, on the guest's behalf, can avoid
@@ -3731,6 +3732,13 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
steal += current->sched_info.run_delay -
vcpu->arch.st.last_steal;
vcpu->arch.st.last_steal = current->sched_info.run_delay;
+ /*
+ * Include the time that the host was suspended in steal time.
+ * Note that the case of a suspend happening during a VM migration
+ * might not be accounted.
+ */
+ steal += suspend_ns - vcpu->arch.st.last_suspend_ns;
+ vcpu->arch.st.last_suspend_ns = suspend_ns;
unsafe_put_user(steal, &st->steal, out);

version += 1;
@@ -12299,6 +12307,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
if (r)
goto free_guest_fpu;

+ vcpu->arch.st.last_suspend_ns = kvm_total_suspend_ns();
kvm_xen_init_vcpu(vcpu);
vcpu_load(vcpu);
kvm_set_tsc_khz(vcpu, vcpu->kvm->arch.default_tsc_khz);
--
2.47.1.613.gc27f4b7a9f-goog