linux-kernel - Re: [PATCH] arch/powerpc/kvm: kvmppc_core_vcpu_create

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Date:   Tue, 28 Mar 2023 15:44:02 +0530
From:   Kautuk Consul <kconsul@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Fabiano Rosas <farosas@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arch/powerpc/kvm: kvmppc_core_vcpu_create_hv: check for
 kzalloc failure

On 2023-03-28 20:44:48, Michael Ellerman wrote:
> Kautuk Consul <kconsul@...ux.vnet.ibm.com> writes:
> > kvmppc_vcore_create() might not be able to allocate memory through
> > kzalloc. In that case the kvm->arch.online_vcores shouldn't be
> > incremented.
> 
> I agree that looks wrong.
> 
> Have you tried to test what goes wrong if it fails? It looks like it
> will break the LPCR update, which likely will cause the guest to crash
> horribly.
Not sure about LPCR update, but with and without the patch qemu exits
and so the kvm context is pulled down fine.
> 
> You could use CONFIG_FAIL_SLAB and fail-nth etc. to fail just one
> allocation for a guest. Or probably easier to just hack the code to fail
> the 4th time it's called using a static counter.
I am using live debug and I set the r3 return value to 0x0 after the
call to kzalloc.
> 
> Doesn't really matter but could be interesting.
With and without this patch qemu quits with:
qemu-system-ppc64: kvm_init_vcpu: kvm_get_vcpu failed (0): Cannot allocate memory

That's because qemu will shut down when any vcpu is not able
to be allocated.
> 
> > Add a check for kzalloc failure and return with -ENOMEM from
> > kvmppc_core_vcpu_create_hv().
> >
> > Signed-off-by: Kautuk Consul <kconsul@...ux.vnet.ibm.com>
> > ---
> >  arch/powerpc/kvm/book3s_hv.c | 10 +++++++---
> >  1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> > index 6ba68dd6190b..e29ee755c920 100644
> > --- a/arch/powerpc/kvm/book3s_hv.c
> > +++ b/arch/powerpc/kvm/book3s_hv.c
> > @@ -2968,13 +2968,17 @@ static int kvmppc_core_vcpu_create_hv(struct kvm_vcpu *vcpu)
> >  			pr_devel("KVM: collision on id %u", id);
> >  			vcore = NULL;
> >  		} else if (!vcore) {
> > +			vcore = kvmppc_vcore_create(kvm,
> > +					id & ~(kvm->arch.smt_mode - 1));
> 
> That line doesn't need to be wrapped, we allow 90 columns.
> 
> > +			if (unlikely(!vcore)) {
> > +				mutex_unlock(&kvm->lock);
> > +				return -ENOMEM;
> > +			}
> 
> Rather than introducing a new return point here, I think it would be
> preferable to use the existing !vcore case below.
> 
> >  			/*
> >  			 * Take mmu_setup_lock for mutual exclusion
> >  			 * with kvmppc_update_lpcr().
> >  			 */
> > -			err = -ENOMEM;
> > -			vcore = kvmppc_vcore_create(kvm,
> > -					id & ~(kvm->arch.smt_mode - 1));
> 
> So leave that as is (maybe move the comment down).
> 
> And wrap the below in:
> 
>  +                      if (vcore) {
> 
> >  			mutex_lock(&kvm->arch.mmu_setup_lock);
> >  			kvm->arch.vcores[core] = vcore;
> >  			kvm->arch.online_vcores++;
>  			
>  			mutex_unlock(&kvm->arch.mmu_setup_lock);
>  +                      }
> 		}
> 	}
> 
> Meaning the vcore == NULL case will fall through to here and return via
> this existing path:
> 
> 	mutex_unlock(&kvm->lock);
> 
> 	if (!vcore)
> 		return err;
> 
> 
> cheers