linux-kernel - Re: [PATCH v3 02/14] KVM: s390: pv: avoid stall notifications for some UVCs

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Date:   Fri, 6 Aug 2021 11:33:25 +0200
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     kvm@...r.kernel.org, cohuck@...hat.com, borntraeger@...ibm.com,
        frankja@...ux.ibm.com, thuth@...hat.com, pasic@...ux.ibm.com,
        linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ulrich.Weigand@...ibm.com
Subject: Re: [PATCH v3 02/14] KVM: s390: pv: avoid stall notifications for
 some UVCs

On Fri, 6 Aug 2021 09:30:04 +0200
David Hildenbrand <david@...hat.com> wrote:

> On 04.08.21 17:40, Claudio Imbrenda wrote:
> > Improve make_secure_pte to avoid stalls when the system is heavily
> > overcommitted. This was especially problematic in
> > kvm_s390_pv_unpack, because of the loop over all pages that needed
> > unpacking.
> > 
> > Also fix kvm_s390_pv_init_vm to avoid stalls when the system is
> > heavily overcommitted.  
> 
> I suggest splitting this change into a separate patch and adding a
> bit more meat to the description why using the other variant is
> possible in the called context. I was kind of surprise to find that
> change buried in this patch.
> 
> Then, you can give both patches a more descriptive patch subject.

fair enough, I'll split them

> > 
> > Signed-off-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>
> > Fixes: 214d9bbcd3a672 ("s390/mm: provide memory management
> > functions for protected KVM guests") ---
> >   arch/s390/kernel/uv.c | 29 +++++++++++++++++++++++------
> >   arch/s390/kvm/pv.c    |  2 +-
> >   2 files changed, 24 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
> > index aeb0a15bcbb7..68a8fbafcb9c 100644
> > --- a/arch/s390/kernel/uv.c
> > +++ b/arch/s390/kernel/uv.c
> > @@ -180,7 +180,7 @@ static int make_secure_pte(pte_t *ptep,
> > unsigned long addr, {
> >   	pte_t entry = READ_ONCE(*ptep);
> >   	struct page *page;
> > -	int expected, rc = 0;
> > +	int expected, cc = 0;
> >   
> >   	if (!pte_present(entry))
> >   		return -ENXIO;
> > @@ -196,12 +196,25 @@ static int make_secure_pte(pte_t *ptep,
> > unsigned long addr, if (!page_ref_freeze(page, expected))
> >   		return -EBUSY;
> >   	set_bit(PG_arch_1, &page->flags);
> > -	rc = uv_call(0, (u64)uvcb);
> > +	/*
> > +	 * If the UVC does not succeed or fail immediately, we
> > don't want to
> > +	 * loop for long, or we might get stall notifications.
> > +	 * On the other hand, this is a complex scenario and we
> > are holding a lot of
> > +	 * locks, so we can't easily sleep and reschedule. We try
> > only once,
> > +	 * and if the UVC returned busy or partial completion, we
> > return
> > +	 * -EAGAIN and we let the callers deal with it.
> > +	 */
> > +	cc = __uv_call(0, (u64)uvcb);
> >   	page_ref_unfreeze(page, expected);
> > -	/* Return -ENXIO if the page was not mapped, -EINVAL
> > otherwise */
> > -	if (rc)
> > -		rc = uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
> > -	return rc;
> > +	/*
> > +	 * Return -ENXIO if the page was not mapped, -EINVAL for
> > other errors.
> > +	 * If busy or partially completed, return -EAGAIN.
> > +	 */
> > +	if (cc == UVC_CC_OK)
> > +		return 0;
> > +	else if (cc == UVC_CC_BUSY || cc == UVC_CC_PARTIAL)
> > +		return -EAGAIN;
> > +	return uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
> >   }  
> 
> That looks conceptually like the right thing to me.
>