From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 05196C433EF for ; Tue, 8 Mar 2022 09:53:53 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S241479AbiCHJyr (ORCPT ); Tue, 8 Mar 2022 04:54:47 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38800 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235300AbiCHJyq (ORCPT ); Tue, 8 Mar 2022 04:54:46 -0500 Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com [148.163.156.1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 096E436691; Tue, 8 Mar 2022 01:53:50 -0800 (PST) Received: from pps.filterd (m0098409.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.1.2/8.16.1.2) with SMTP id 2289igdF023114; Tue, 8 Mar 2022 09:53:49 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ibm.com; h=message-id : date : mime-version : subject : to : cc : references : from : in-reply-to : content-type : content-transfer-encoding; s=pp1; bh=vQVV7PfNsJzK58f284nM7Fiw51dcMsHARivNMjKwCOU=; b=MeMbm+Fu3Sy23eAKuHdZjbqQKUMK7vyY/1EAEN6cUlcwJKzKl2NQX79B1hyNKlfL/PHO 7EsTMVEHKT+ca2JaEDj7nloa5nX9J5pPgsN7Vo1w9ai6EcM99/LbT2TdO9Ufp6YHWXI3 ySsYCaR3qMUw/7KPgcDgWfRRSTa/c04GVUAgQSm6rOMRdBlSHBv3E+ORY7W98vOgK94Q MEjpxI48/aNMwmPYUFV5h6uveu0RAsTBLtvl65p4fGdiM7yqxUTS1S0LzpGfTFBICjJ3 tuQfzxZo4oXSTh0J02K8NKUPeYp/VKzT7f4jmBmRbWCt4HikdR/FQl0ro9LvekMsRkWa fw== Received: from pps.reinject (localhost [127.0.0.1]) by mx0a-001b2d01.pphosted.com with ESMTP id 3enx3kyy0p-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 08 Mar 2022 09:53:49 +0000 Received: from m0098409.ppops.net (m0098409.ppops.net [127.0.0.1]) by pps.reinject (8.16.0.43/8.16.0.43) with SMTP id 2288mZIg012993; Tue, 8 Mar 2022 09:53:49 GMT Received: from ppma04fra.de.ibm.com (6a.4a.5195.ip4.static.sl-reverse.com [149.81.74.106]) by mx0a-001b2d01.pphosted.com with ESMTP id 3enx3kyy09-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 08 Mar 2022 09:53:49 +0000 Received: from pps.filterd (ppma04fra.de.ibm.com [127.0.0.1]) by ppma04fra.de.ibm.com (8.16.1.2/8.16.1.2) with SMTP id 2289rOQ0017827; Tue, 8 Mar 2022 09:53:46 GMT Received: from b06avi18626390.portsmouth.uk.ibm.com (b06avi18626390.portsmouth.uk.ibm.com [9.149.26.192]) by ppma04fra.de.ibm.com with ESMTP id 3ekyg962cn-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Tue, 08 Mar 2022 09:53:46 +0000 Received: from d06av25.portsmouth.uk.ibm.com (d06av25.portsmouth.uk.ibm.com [9.149.105.61]) by b06avi18626390.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id 2289gYrB23986648 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Tue, 8 Mar 2022 09:42:34 GMT Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 33D1A11C05B; Tue, 8 Mar 2022 09:53:43 +0000 (GMT) Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D630E11C054; Tue, 8 Mar 2022 09:53:42 +0000 (GMT) Received: from [9.171.93.186] (unknown [9.171.93.186]) by d06av25.portsmouth.uk.ibm.com (Postfix) with ESMTP; Tue, 8 Mar 2022 09:53:42 +0000 (GMT) Message-ID: Date: Tue, 8 Mar 2022 10:53:42 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.5.0 Subject: Re: [PATCH v1 1/1] KVM: s390x: fix SCK locking Content-Language: en-US To: Claudio Imbrenda , kvm@vger.kernel.org Cc: linux-s390@vger.kernel.org, frankja@linux.ibm.com, thuth@redhat.com, mimu@linux.ibm.com References: <20220301143340.111129-1-imbrenda@linux.ibm.com> From: Christian Borntraeger In-Reply-To: <20220301143340.111129-1-imbrenda@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: 7U4BUtOMbmqVgD2FuU7gf7sFo5Lx0vnH X-Proofpoint-GUID: H6wkK5_miDJOgshZ4FdTx1IpMH3yAQeM X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.816,Hydra:6.0.425,FMLib:17.11.64.514 definitions=2022-03-08_03,2022-03-04_01,2022-02-23_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 clxscore=1015 impostorscore=0 mlxscore=0 lowpriorityscore=0 phishscore=0 suspectscore=0 mlxlogscore=999 priorityscore=1501 spamscore=0 bulkscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2202240000 definitions=main-2203080049 Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org Am 01.03.22 um 15:33 schrieb Claudio Imbrenda: > When handling the SCK instruction, the kvm lock is taken, even though > the vcpu lock is already being held. The normal locking order is kvm > lock first and then vcpu lock. This is can (and in some circumstances > does) lead to deadlocks. > > The function kvm_s390_set_tod_clock is called both by the SCK handler > and by some IOCTLs to set the clock. The IOCTLs will not hold the vcpu > lock, so they can safely take the kvm lock. The SCK handler holds the > vcpu lock, but will also somehow need to acquire the kvm lock without > relinquishing the vcpu lock. > > The solution is to factor out the code to set the clock, and provide > two wrappers. One is called like the original function and does the > locking, the other is called kvm_s390_try_set_tod_clock and uses > trylock to try to acquire the kvm lock. This new wrapper is then used > in the SCK handler. If locking fails, -EAGAIN is returned, which is > eventually propagated to userspace, thus also freeing the vcpu lock and > allowing for forward progress. > > This is not the most efficient or elegant way to solve this issue, but > the SCK instruction is deprecated and its performance is not critical. > > The goal of this patch is just to provide a simple but correct way to > fix the bug. > > Fixes: 6a3f95a6b04c ("KVM: s390: Intercept SCK instruction") > Signed-off-by: Claudio Imbrenda Reviewed-by: Christian Borntraeger > --- > arch/s390/kvm/kvm-s390.c | 19 ++++++++++++++++--- > arch/s390/kvm/kvm-s390.h | 4 ++-- > arch/s390/kvm/priv.c | 14 +++++++++++++- > 3 files changed, 31 insertions(+), 6 deletions(-) > > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c > index 2296b1ff1e02..4e3db4004bfd 100644 > --- a/arch/s390/kvm/kvm-s390.c > +++ b/arch/s390/kvm/kvm-s390.c > @@ -3869,14 +3869,12 @@ static int kvm_s390_handle_requests(struct kvm_vcpu *vcpu) > return 0; > } > > -void kvm_s390_set_tod_clock(struct kvm *kvm, > - const struct kvm_s390_vm_tod_clock *gtod) > +static void __kvm_s390_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod) > { > struct kvm_vcpu *vcpu; > union tod_clock clk; > unsigned long i; > > - mutex_lock(&kvm->lock); > preempt_disable(); > > store_tod_clock_ext(&clk); > @@ -3897,7 +3895,22 @@ void kvm_s390_set_tod_clock(struct kvm *kvm, > > kvm_s390_vcpu_unblock_all(kvm); > preempt_enable(); > +} > + > +void kvm_s390_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod) > +{ > + mutex_lock(&kvm->lock); > + __kvm_s390_set_tod_clock(kvm, gtod); > + mutex_unlock(&kvm->lock); > +} > + > +int kvm_s390_try_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod) > +{ > + if (!mutex_trylock(&kvm->lock)) > + return 0; > + __kvm_s390_set_tod_clock(kvm, gtod); > mutex_unlock(&kvm->lock); > + return 1; > } > > /** > diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h > index 098831e815e6..f2c910763d7f 100644 > --- a/arch/s390/kvm/kvm-s390.h > +++ b/arch/s390/kvm/kvm-s390.h > @@ -349,8 +349,8 @@ int kvm_s390_handle_sigp(struct kvm_vcpu *vcpu); > int kvm_s390_handle_sigp_pei(struct kvm_vcpu *vcpu); > > /* implemented in kvm-s390.c */ > -void kvm_s390_set_tod_clock(struct kvm *kvm, > - const struct kvm_s390_vm_tod_clock *gtod); > +void kvm_s390_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod); > +int kvm_s390_try_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod); > long kvm_arch_fault_in_page(struct kvm_vcpu *vcpu, gpa_t gpa, int writable); > int kvm_s390_store_status_unloaded(struct kvm_vcpu *vcpu, unsigned long addr); > int kvm_s390_vcpu_store_status(struct kvm_vcpu *vcpu, unsigned long addr); > diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c > index 417154b314a6..7f3e7990ef82 100644 > --- a/arch/s390/kvm/priv.c > +++ b/arch/s390/kvm/priv.c > @@ -102,7 +102,19 @@ static int handle_set_clock(struct kvm_vcpu *vcpu) > return kvm_s390_inject_prog_cond(vcpu, rc); > > VCPU_EVENT(vcpu, 3, "SCK: setting guest TOD to 0x%llx", gtod.tod); > - kvm_s390_set_tod_clock(vcpu->kvm, >od); > + /* > + * To set the TOD clock we need to take the kvm lock, but we are > + * already holding the vcpu lock, and the usual lock order is the > + * opposite. Therefore we use trylock instead of lock, and if the > + * kvm lock cannot be taken, we retry the instruction and return > + * -EAGAIN to userspace, thus freeing the vcpu lock. > + * The SCK instruction is considered legacy and at this point it's > + * not worth the effort to find a nicer solution. > + */ > + if (!kvm_s390_try_set_tod_clock(vcpu->kvm, >od)) { > + kvm_s390_retry_instr(vcpu); > + return -EAGAIN; > + } > > kvm_s390_set_psw_cc(vcpu, 0); > return 0;