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[91.12.97.4]) by smtp.gmail.com with ESMTPSA id b6sm10178492wrn.9.2021.08.06.00.30.04 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 06 Aug 2021 00:30:05 -0700 (PDT) Subject: Re: [PATCH v3 02/14] KVM: s390: pv: avoid stall notifications for some UVCs To: Claudio Imbrenda , kvm@vger.kernel.org Cc: cohuck@redhat.com, borntraeger@de.ibm.com, frankja@linux.ibm.com, thuth@redhat.com, pasic@linux.ibm.com, linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org, Ulrich.Weigand@de.ibm.com References: <20210804154046.88552-1-imbrenda@linux.ibm.com> <20210804154046.88552-3-imbrenda@linux.ibm.com> From: David Hildenbrand Organization: Red Hat Message-ID: <7ef282bb-d90f-9cbb-678b-4293790fb8c6@redhat.com> Date: Fri, 6 Aug 2021 09:30:04 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210804154046.88552-3-imbrenda@linux.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org 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. > > 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/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. -- Thanks, David / dhildenb