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Wed, 6 Oct 2021 16:14:47 +0000 (GMT) Received: from p-imbrenda (unknown [9.145.10.200]) by d06av26.portsmouth.uk.ibm.com (Postfix) with ESMTP; Wed, 6 Oct 2021 16:14:47 +0000 (GMT) Date: Wed, 6 Oct 2021 18:14:44 +0200 From: Claudio Imbrenda To: Christian Borntraeger Cc: kvm@vger.kernel.org, cohuck@redhat.com, frankja@linux.ibm.com, thuth@redhat.com, pasic@linux.ibm.com, david@redhat.com, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, Ulrich.Weigand@de.ibm.com Subject: Re: [PATCH v5 04/14] KVM: s390: pv: avoid stalls when making pages secure Message-ID: <20211006181444.532a1e43@p-imbrenda> In-Reply-To: References: <20210920132502.36111-1-imbrenda@linux.ibm.com> <20210920132502.36111-5-imbrenda@linux.ibm.com> Organization: IBM X-Mailer: Claws Mail 3.18.0 (GTK+ 2.24.33; x86_64-redhat-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: 0lkMRY8HT1gZjeFdx466V6JfPsUApLI- X-Proofpoint-GUID: POmuNy5O8Vt6ClRrrd3JJu5a7SQe8j3f X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.182.1,Aquarius:18.0.790,Hydra:6.0.391,FMLib:17.0.607.475 definitions=2021-10-06_04,2021-10-06_01,2020-04-07_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 mlxscore=0 suspectscore=0 clxscore=1015 adultscore=0 mlxlogscore=999 spamscore=0 lowpriorityscore=0 bulkscore=0 malwarescore=0 impostorscore=0 phishscore=0 priorityscore=1501 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2109230001 definitions=main-2110060101 Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On Wed, 6 Oct 2021 17:54:00 +0200 Christian Borntraeger wrote: > Am 20.09.21 um 15:24 schrieb Claudio Imbrenda: > > 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. > > > > Due to the locks being held, it was not possible to simply replace > > uv_call with uv_call_sched. A more complex approach was > > needed, in which uv_call is replaced with __uv_call, which does not > > loop. When the UVC needs to be executed again, -EAGAIN is returned, and > > the caller (or its caller) will try again. > > > > When -EAGAIN is returned, the path is the same as when the page is in > > writeback (and the writeback check is also performed, which is > > harmless). > > > > Signed-off-by: Claudio Imbrenda > > Fixes: 214d9bbcd3a672 ("s390/mm: provide memory management functions for protected KVM guests") > > --- > > arch/s390/kernel/uv.c | 29 +++++++++++++++++++++++------ > > arch/s390/kvm/intercept.c | 5 +++++ > > 2 files changed, 28 insertions(+), 6 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; > > } > > > > /* > > @@ -254,6 +267,10 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb) > > mmap_read_unlock(gmap->mm); > > > > if (rc == -EAGAIN) { > > + /* > > + * If we are here because the UVC returned busy or partial > > + * completion, this is just a useless check, but it is safe. > > + */ > > wait_on_page_writeback(page); > > } else if (rc == -EBUSY) { > > /* > > diff --git a/arch/s390/kvm/intercept.c b/arch/s390/kvm/intercept.c > > index 72b25b7cc6ae..47833ade4da5 100644 > > --- a/arch/s390/kvm/intercept.c > > +++ b/arch/s390/kvm/intercept.c > > @@ -516,6 +516,11 @@ static int handle_pv_uvc(struct kvm_vcpu *vcpu) > > */ > > if (rc == -EINVAL) > > return 0; > > + /* > > + * If we got -EAGAIN here, we simply return it. It will eventually > > + * get propagated all the way to userspace, which should then try > > + * again. > > + */ > > This cpoment is new over v4, right? Can this happen often? If not then this is ok > otherwise we should consider your proposal of doing yes, the comment is new. I would expect this to happen only when the system is under load. in any case this is better than busy waiting on the UVC > > if (rc == -EINVAL || rc == -EAGAIN) > > to reduce overhead. > > Anyway,for both ways > > Reviewed-by: Christian Borntraeger > > > return rc; > > } > > > >