From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-1.mimecast.com ([205.139.110.61]:35611 "EHLO us-smtp-delivery-1.mimecast.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726366AbgD2J4I (ORCPT ); Wed, 29 Apr 2020 05:56:08 -0400 Subject: Re: [PATCH v2 08/10] s390x: smp: Wait for sigp completion References: <20200423091013.11587-1-frankja@linux.ibm.com> <20200423091013.11587-9-frankja@linux.ibm.com> <6084d368-86d6-b8fd-d4d3-5e0d72cef590@redhat.com> <18b6f022-81b7-6e0d-996d-3abcffceca41@linux.ibm.com> <8182df06-8190-001d-ad02-ae13fb99ec72@redhat.com> <802601e1-0bc0-faba-b802-2b0e24e3d96b@linux.ibm.com> From: David Hildenbrand Message-ID: <5ef08433-10fd-ccca-eb13-5a93bd462c4c@redhat.com> Date: Wed, 29 Apr 2020 11:55:59 +0200 MIME-Version: 1.0 In-Reply-To: <802601e1-0bc0-faba-b802-2b0e24e3d96b@linux.ibm.com> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: quoted-printable Sender: linux-s390-owner@vger.kernel.org List-ID: To: Janosch Frank , kvm@vger.kernel.org Cc: thuth@redhat.com, linux-s390@vger.kernel.org, borntraeger@de.ibm.com, cohuck@redhat.com On 29.04.20 11:37, Janosch Frank wrote: > On 4/29/20 11:06 AM, David Hildenbrand wrote: >> On 29.04.20 10:57, Janosch Frank wrote: >>> On 4/24/20 1:40 PM, Janosch Frank wrote: >>>> On 4/24/20 12:11 PM, David Hildenbrand wrote: >>>>> On 23.04.20 11:10, Janosch Frank wrote: >>>>>> Sigp orders are not necessarily finished when the processor finish= ed >>>>>> the sigp instruction. We need to poll if the order has been finish= ed >>>>>> before we continue. >>>>>> >>>>>> For (re)start and stop we already use sigp sense running and sigp >>>>>> sense loops. But we still lack completion checks for stop and stor= e >>>>>> status, as well as the cpu resets. >>>>>> >>>>>> Let's add them. >>>>>> >>>>>> Signed-off-by: Janosch Frank >>>>>> Reviewed-by: Cornelia Huck >>>>>> --- >>>>>> lib/s390x/smp.c | 8 ++++++++ >>>>>> lib/s390x/smp.h | 1 + >>>>>> s390x/smp.c | 4 ++++ >>>>>> 3 files changed, 13 insertions(+) >>>>>> >>>>>> diff --git a/lib/s390x/smp.c b/lib/s390x/smp.c >>>>>> index 6ef0335..2555bf4 100644 >>>>>> --- a/lib/s390x/smp.c >>>>>> +++ b/lib/s390x/smp.c >>>>>> @@ -154,6 +154,14 @@ int smp_cpu_start(uint16_t addr, struct psw p= sw) >>>>>> return rc; >>>>>> } >>>>>> =20 >>>>>> +void smp_cpu_wait_for_completion(uint16_t addr) >>>>>> +{ >>>>>> + uint32_t status; >>>>>> + >>>>>> + /* Loops when cc =3D=3D 2, i.e. when the cpu is busy with a sigp= order */ >>>>>> + sigp_retry(1, SIGP_SENSE, 0, &status); >>>>>> +} >>>>>> + >>>>>> int smp_cpu_destroy(uint16_t addr) >>>>>> { >>>>>> struct cpu *cpu; >>>>>> diff --git a/lib/s390x/smp.h b/lib/s390x/smp.h >>>>>> index ce63a89..a8b98c0 100644 >>>>>> --- a/lib/s390x/smp.h >>>>>> +++ b/lib/s390x/smp.h >>>>>> @@ -45,6 +45,7 @@ int smp_cpu_restart(uint16_t addr); >>>>>> int smp_cpu_start(uint16_t addr, struct psw psw); >>>>>> int smp_cpu_stop(uint16_t addr); >>>>>> int smp_cpu_stop_store_status(uint16_t addr); >>>>>> +void smp_cpu_wait_for_completion(uint16_t addr); >>>>>> int smp_cpu_destroy(uint16_t addr); >>>>>> int smp_cpu_setup(uint16_t addr, struct psw psw); >>>>>> void smp_teardown(void); >>>>>> diff --git a/s390x/smp.c b/s390x/smp.c >>>>>> index 7462211..48321f4 100644 >>>>>> --- a/s390x/smp.c >>>>>> +++ b/s390x/smp.c >>>>>> @@ -75,6 +75,7 @@ static void test_stop_store_status(void) >>>>>> lc->prefix_sa =3D 0; >>>>>> lc->grs_sa[15] =3D 0; >>>>>> smp_cpu_stop_store_status(1); >>>>>> + smp_cpu_wait_for_completion(1); >>>>>> mb(); >>>>>> report(lc->prefix_sa =3D=3D (uint32_t)(uintptr_t)cpu->lowcore, "= prefix"); >>>>>> report(lc->grs_sa[15], "stack"); >>>>>> @@ -85,6 +86,7 @@ static void test_stop_store_status(void) >>>>>> lc->prefix_sa =3D 0; >>>>>> lc->grs_sa[15] =3D 0; >>>>>> smp_cpu_stop_store_status(1); >>>>> >>>>> Just curious: Would it make sense to add that inside >>>>> smp_cpu_stop_store_status() instead? >>>>> >>>> >>>> I think so, we also wait for stop and start to finish, so why not fo= r >>>> this order code. >>>> >>> >>> I've moved the waiting into the smp library and now the prefix check = for >>> stop and store status fails every so often if executed repeatedly. >>> >>> I've tried making the lc ptr volatile, a print of the prefix before t= he >>> report seems to fix the issue, a print after the report still shows t= he >>> issue but according to the print both values are the same. >>> >>> I'm currently at a loss... >> >> Are you missing a barrier() somewhere? >> >=20 > Maybe, but the question is where? >=20 > There's already one before the report: > smp_cpu_stop_store_status(1); > mb(); > report(lc->prefix_sa =3D=3D (uint32_t)(uintptr_t)cpu->lowcore, "prefix"= ); The issue here is: SIGP_SENSE is always handled in the kernel for KVM. Meaning, it will complete even before the target CPU executed the stop and store (in QEMU)= . Reading the PoP: "One of the following conditions exists at the addressed CPU: ... A previously issued stop- and-store-status ... has been accepted by the addressed CPU, and execution of the func- tion requested by the order has not yet been completed. "If the currently specified order is sense ... then the order is rejected, and condition code 2 is set." So, in case of KVM, SENSE does not wait for completion of the previous order. I remember that was a performance improvements, because we wanted to avoid going to user space just to sense if another CPU is running. (and I remember that the documentation was inconsistent) Let me guess, under TCG it works all the time? --=20 Thanks, David / dhildenb