linux-kernel - Re: [PATCH v3 15/23] KVM: s390: Add helper functions for fault handling

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Message-ID: <20251119120609.4a4bf008@p-imbrenda>
Date: Wed, 19 Nov 2025 12:06:09 +0100
From: Claudio Imbrenda <imbrenda@...ux.ibm.com>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, borntraeger@...ibm.com,
        frankja@...ux.ibm.com, nsg@...ux.ibm.com, nrb@...ux.ibm.com,
        seiden@...ux.ibm.com, schlameuss@...ux.ibm.com, svens@...ux.ibm.com,
        agordeev@...ux.ibm.com, gor@...ux.ibm.com, david@...hat.com,
        gerald.schaefer@...ux.ibm.com
Subject: Re: [PATCH v3 15/23] KVM: s390: Add helper functions for fault
 handling

On Tue, 18 Nov 2025 16:10:05 +0100
Heiko Carstens <hca@...ux.ibm.com> wrote:

> On Thu, Nov 06, 2025 at 05:11:09PM +0100, Claudio Imbrenda wrote:
> > Add some helper functions for handling multiple guest faults at the
> > same time.
> > 
> > This will be needed for VSIE, where a nested guest access also needs to
> > access all the page tables that map it.
> > 
> > Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> > ---
> >  arch/s390/include/asm/kvm_host.h |   1 +
> >  arch/s390/kvm/Makefile           |   2 +-
> >  arch/s390/kvm/faultin.c          | 148 +++++++++++++++++++++++++++++++
> >  arch/s390/kvm/faultin.h          |  92 +++++++++++++++++++
> >  arch/s390/kvm/kvm-s390.c         |   2 +-
> >  arch/s390/kvm/kvm-s390.h         |   2 +
> >  6 files changed, 245 insertions(+), 2 deletions(-)
> >  create mode 100644 arch/s390/kvm/faultin.c
> >  create mode 100644 arch/s390/kvm/faultin.h  
> 
> ...
> 
> > +int kvm_s390_faultin_gfn(struct kvm_vcpu *vcpu, struct kvm *kvm, struct guest_fault *f)
> > +{  
> 
> ...
> 
> > +		scoped_guard(read_lock, &kvm->mmu_lock) {
> > +			if (!mmu_invalidate_retry_gfn(kvm, inv_seq, f->gfn)) {
> > +				f->valid = true;
> > +				rc = gmap_link(mc, kvm->arch.gmap, f);
> > +				kvm_release_faultin_page(kvm, f->page, !!rc, f->write_attempt);
> > +				f->page = NULL;
> > +			}
> > +		}
> > +		kvm_release_faultin_page(kvm, f->page, true, false);
> > +
> > +		if (rc == -ENOMEM) {
> > +			rc = kvm_s390_mmu_cache_topup(mc);  
> 
> If I'm not mistaken then gmap_link() -> dat_link() maps the possible -ENOMEM
> return value of dat_entry() to -EAGAIN. So the case where -ENOMEM leads to a
> kvm_s390_mmu_cache_topup() call will never happen.

oops, you're right

I'll fix dat_link() accordingly