Re: [PATCH 1/1] KVM: s390: index kvm->arch.idle_mask by vcpu_idx

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On 27.08.21 16:06, Claudio Imbrenda wrote:
On Fri, 27 Aug 2021 14:54:29 +0200
Halil Pasic <pasic@xxxxxxxxxxxxx> wrote:

While in practice vcpu->vcpu_idx ==  vcpu->vcp_id is often true,
it may not always be, and we must not rely on this.

why?

maybe add a simple explanation of why vcpu_idx and vcpu_id can be
different, namely:
KVM decides the vcpu_idx, userspace decides the vcpu_id, thus the two
might not match


Currently kvm->arch.idle_mask is indexed by vcpu_id, which implies
that code like
for_each_set_bit(vcpu_id, kvm->arch.idle_mask, online_vcpus) {
                 vcpu = kvm_get_vcpu(kvm, vcpu_id);

you can also add a sentence to clarify that kvm_get_vcpu expects an
vcpu_idx, not an vcpu_id.

		do_stuff(vcpu);

I will modify the patch description accordingly before sending to Paolo.
Thanks for noticing.


}
is not legit. The trouble is, we do actually use kvm->arch.idle_mask
like this. To fix this problem we have two options. Either use
kvm_get_vcpu_by_id(vcpu_id), which would loop to find the right vcpu_id,
or switch to indexing via vcpu_idx. The latter is preferable for obvious
reasons.

Let us make switch from indexing kvm->arch.idle_mask by vcpu_id to
indexing it by vcpu_idx.  To keep gisa_int.kicked_mask indexed by the
same index as idle_mask lets make the same change for it as well.

Signed-off-by: Halil Pasic <pasic@xxxxxxxxxxxxx>
Fixes: 1ee0bc559dc3 ("KVM: s390: get rid of local_int array")

otherwise looks good to me.

Reviewed-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>

Cc: <stable@xxxxxxxxxxxxxxx> # 3.15+
---
  arch/s390/include/asm/kvm_host.h |  1 +
  arch/s390/kvm/interrupt.c        | 12 ++++++------
  arch/s390/kvm/kvm-s390.c         |  2 +-
  arch/s390/kvm/kvm-s390.h         |  2 +-
  4 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/arch/s390/include/asm/kvm_host.h b/arch/s390/include/asm/kvm_host.h
index 161a9e12bfb8..630eab0fa176 100644
--- a/arch/s390/include/asm/kvm_host.h
+++ b/arch/s390/include/asm/kvm_host.h
@@ -957,6 +957,7 @@ struct kvm_arch{
  	atomic64_t cmma_dirty_pages;
  	/* subset of available cpu features enabled by user space */
  	DECLARE_BITMAP(cpu_feat, KVM_S390_VM_CPU_FEAT_NR_BITS);
+	/* indexed by vcpu_idx */
  	DECLARE_BITMAP(idle_mask, KVM_MAX_VCPUS);
  	struct kvm_s390_gisa_interrupt gisa_int;
  	struct kvm_s390_pv pv;
diff --git a/arch/s390/kvm/interrupt.c b/arch/s390/kvm/interrupt.c
index d548d60caed2..16256e17a544 100644
--- a/arch/s390/kvm/interrupt.c
+++ b/arch/s390/kvm/interrupt.c
@@ -419,13 +419,13 @@ static unsigned long deliverable_irqs(struct kvm_vcpu *vcpu)
  static void __set_cpu_idle(struct kvm_vcpu *vcpu)
  {
  	kvm_s390_set_cpuflags(vcpu, CPUSTAT_WAIT);
-	set_bit(vcpu->vcpu_id, vcpu->kvm->arch.idle_mask);
+	set_bit(kvm_vcpu_get_idx(vcpu), vcpu->kvm->arch.idle_mask);
  }
static void __unset_cpu_idle(struct kvm_vcpu *vcpu)
  {
  	kvm_s390_clear_cpuflags(vcpu, CPUSTAT_WAIT);
-	clear_bit(vcpu->vcpu_id, vcpu->kvm->arch.idle_mask);
+	clear_bit(kvm_vcpu_get_idx(vcpu), vcpu->kvm->arch.idle_mask);
  }
static void __reset_intercept_indicators(struct kvm_vcpu *vcpu)
@@ -3050,18 +3050,18 @@ int kvm_s390_get_irq_state(struct kvm_vcpu *vcpu, __u8 __user *buf, int len)
static void __airqs_kick_single_vcpu(struct kvm *kvm, u8 deliverable_mask)
  {
-	int vcpu_id, online_vcpus = atomic_read(&kvm->online_vcpus);
+	int vcpu_idx, online_vcpus = atomic_read(&kvm->online_vcpus);
  	struct kvm_s390_gisa_interrupt *gi = &kvm->arch.gisa_int;
  	struct kvm_vcpu *vcpu;
- for_each_set_bit(vcpu_id, kvm->arch.idle_mask, online_vcpus) {
-		vcpu = kvm_get_vcpu(kvm, vcpu_id);
+	for_each_set_bit(vcpu_idx, kvm->arch.idle_mask, online_vcpus) {
+		vcpu = kvm_get_vcpu(kvm, vcpu_idx);
  		if (psw_ioint_disabled(vcpu))
  			continue;
  		deliverable_mask &= (u8)(vcpu->arch.sie_block->gcr[6] >> 24);
  		if (deliverable_mask) {
  			/* lately kicked but not yet running */
-			if (test_and_set_bit(vcpu_id, gi->kicked_mask))
+			if (test_and_set_bit(vcpu_idx, gi->kicked_mask))
  				return;
  			kvm_s390_vcpu_wakeup(vcpu);
  			return;
diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 4527ac7b5961..8580543c5bc3 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -4044,7 +4044,7 @@ static int vcpu_pre_run(struct kvm_vcpu *vcpu)
  		kvm_s390_patch_guest_per_regs(vcpu);
  	}
- clear_bit(vcpu->vcpu_id, vcpu->kvm->arch.gisa_int.kicked_mask);
+	clear_bit(kvm_vcpu_get_idx(vcpu), vcpu->kvm->arch.gisa_int.kicked_mask);
vcpu->arch.sie_block->icptcode = 0;
  	cpuflags = atomic_read(&vcpu->arch.sie_block->cpuflags);
diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
index 9fad25109b0d..ecd741ee3276 100644
--- a/arch/s390/kvm/kvm-s390.h
+++ b/arch/s390/kvm/kvm-s390.h
@@ -79,7 +79,7 @@ static inline int is_vcpu_stopped(struct kvm_vcpu *vcpu)
static inline int is_vcpu_idle(struct kvm_vcpu *vcpu)
  {
-	return test_bit(vcpu->vcpu_id, vcpu->kvm->arch.idle_mask);
+	return test_bit(kvm_vcpu_get_idx(vcpu), vcpu->kvm->arch.idle_mask);
  }
static inline int kvm_is_ucontrol(struct kvm *kvm)

base-commit: 77dd11439b86e3f7990e4c0c9e0b67dca82750ba




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