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

[Halil Pasic <pasic@linux.ibm.com>]

On Mon, 31 Jan 2022 11:13:18 +0100
Petr Tesařík <ptesarik@suse.cz> wrote:

> Hi Halil,
> 
> Dne 27. 08. 21 v 14:54 Halil Pasic napsal(a):
> > While in practice vcpu->vcpu_idx ==  vcpu->vcp_id is often true,
> > it may not always be, and we must not rely on this.
> > 
> > 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);
> > 		do_stuff(vcpu);
> > }
> > 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.  
> 
> I'm just backporting this fix to SLES 12 SP5, and I've noticed that 
> there is still this code in __floating_irq_kick():
> 
> 	/* find idle VCPUs first, then round robin */
> 	sigcpu = find_first_bit(fi->idle_mask, online_vcpus);
> /* ... round robin loop removed ...
> 	dst_vcpu = kvm_get_vcpu(kvm, sigcpu);
> 
> It seems to me that this does exactly the thing that is not legit, but 
> I'm no expert. Did I miss something?
> 

We made that legit by making the N-th bit in idle_mask correspond to the
vcpu whose vcpu_idx == N. The second argument of kvm_get_vcpu() is the
vcpu_idx. IMHO that ain't super-intuitive because it ain't spelled out.

So before this was a mismatch (with a vcpu_id based bitmap we would have
to use kvm_get_vcpu_by_id()), and with this patch applied this code
becomes legit because both idle_mask and kvm_get_vcpu() operate with
vcpu_idx.

Does that make sense?

I'm sorry the commit message did not convey this clearly enough...

Regards,
Halil


> Petr T
> 
> > 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@linux.ibm.com>
> > Fixes: 1ee0bc559dc3 ("KVM: s390: get rid of local_int array")
> > Cc: <stable@vger.kernel.org> # 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