Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Roman Kagan <rkagan@virtuozzo.com>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
CC:     "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?iso-8859-2?Q?Radim_Kr=E8m=E1=F8?= <rkrcmar@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86/kvm/hyper-v: avoid spurious pending stimer on vCPU
 init
Thread-Topic: [PATCH] x86/kvm/hyper-v: avoid spurious pending stimer on vCPU
 init
Thread-Index: AQHU2cAg1Ir75Ld2kEirRD2GDjkNfaYRKFGA
Date:   Mon, 18 Mar 2019 09:35:27 +0000
Message-ID: <20190318093523.GA11912@rkaganb.sw.ru>
References: <20190313171342.12814-1-vkuznets@redhat.com>
In-Reply-To: <20190313171342.12814-1-vkuznets@redhat.com>
Accept-Language: en-US, ru-RU
Content-Language: en-US
user-agent: Mutt/1.11.3 (2019-02-01)
mail-followup-to: =?iso-8859-2?Q?Roman_Kagan_<rkagan@virtuozzo.com>,=09Vitaly_Kuznetsov_<vk?=
 =?iso-8859-2?Q?uznets@redhat.com>,_kvm@vger.kernel.org,=09Paolo_Bonzini_<?=
 =?iso-8859-2?Q?pbonzini@redhat.com>,=09Radim_Kr=E8m=E1=F8_<rkrcmar@redhat?=
 =?iso-8859-2?Q?.com>,=09linux-kernel@vger.kernel.org?=
x-ms-exchange-messagesentrepresentingtype: 1
x-ms-publictraffictype: Email
x-ms-office365-filtering-correlation-id: bcd0b9b6-102a-4f4a-07d3-08d6ab850d99
x-microsoft-antispam: BCL:0;PCL:0;RULEID:(2390118)(7020095)(4652040)(8989299)(4534185)(4627221)(201703031133081)(201702281549075)(8990200)(5600127)(711020)(4605104)(2017052603328)(7153060)(7193020);SRVR:DB8PR08MB5099;
x-ms-traffictypediagnostic: DB8PR08MB5099:|DB8PR08MB5099:
x-microsoft-antispam-prvs: <DB8PR08MB509956B99FB5E0711A3E3EE6C9470@DB8PR08MB5099.eurprd08.prod.outlook.com>
x-forefront-prvs: 098076C36C
x-forefront-antispam-report: SFV:SPM;SFS:(10019020)(39840400004)(396003)(376002)(366004)(346002)(136003)(189003)(199004)(81166006)(97736004)(81156014)(71200400001)(71190400001)(86362001)(6916009)(486006)(25786009)(36756003)(26005)(8676002)(14444005)(256004)(68736007)(478600001)(14454004)(446003)(8936002)(1076003)(2906002)(6116002)(3846002)(5660300002)(4326008)(11346002)(52116002)(102836004)(9686003)(229853002)(476003)(316002)(386003)(33656002)(6506007)(6436002)(6512007)(7736002)(305945005)(186003)(106356001)(66066001)(6486002)(99286004)(58126008)(54906003)(6246003)(76176011)(105586002)(53936002)(30126002);DIR:OUT;SFP:1501;SCL:5;SRVR:DB8PR08MB5099;H:DB8PR08MB4092.eurprd08.prod.outlook.com;FPR:;SPF:None;LANG:en;PTR:InfoNoRecords;MX:1;A:1;
received-spf: None (protection.outlook.com: virtuozzo.com does not designate
 permitted sender hosts)
x-ms-exchange-senderadcheck: 1
x-microsoft-antispam-message-info: 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
Content-Type: text/plain; charset="iso-8859-2"
Content-ID: <F3B0B5C4FA4E68448A0826967862B17E@eurprd08.prod.outlook.com>
Content-Transfer-Encoding: quoted-printable
MIME-Version: 1.0
X-OriginatorOrg: virtuozzo.com
X-MS-Exchange-CrossTenant-Network-Message-Id: bcd0b9b6-102a-4f4a-07d3-08d6ab850d99
X-MS-Exchange-CrossTenant-originalarrivaltime: 18 Mar 2019 09:35:27.4812
 (UTC)
X-MS-Exchange-CrossTenant-fromentityheader: Hosted
X-MS-Exchange-CrossTenant-id: 0bc7f26d-0264-416e-a6fc-8352af79c58f
X-MS-Exchange-CrossTenant-mailboxtype: HOSTED
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DB8PR08MB5099
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Mar 13, 2019 at 06:13:42PM +0100, Vitaly Kuznetsov wrote:
> When userspace initializes guest vCPUs it may want to zero all supported
> MSRs including Hyper-V related ones including HV_X64_MSR_STIMERn_CONFIG/
> HV_X64_MSR_STIMERn_COUNT. With commit f3b138c5d89a ("kvm/x86: Update SynI=
C
> timers on guest entry only") we began doing stimer_mark_pending()
> unconditionally on every config change.
>=20
> The issue I'm observing manifests itself as following:
> - Qemu writes 0 to STIMERn_{CONFIG,COUNT} MSRs and marks all stimers as
>   pending in stimer_pending_bitmap, arms KVM_REQ_HV_STIMER;
> - kvm_hv_has_stimer_pending() starts returning true;
> - kvm_vcpu_has_events() starts returning true;
> - kvm_arch_vcpu_runnable() starts returning true;
> - when kvm_arch_vcpu_ioctl_run() gets into
>   (vcpu->arch.mp_state =3D=3D KVM_MP_STATE_UNINITIALIZED) case:
>   - kvm_vcpu_block() gets in 'kvm_vcpu_check_block(vcpu) < 0' and returns
>     immediately, avoiding normal wait path;
>   - -EAGAIN is returned from kvm_arch_vcpu_ioctl_run() immediately forcin=
g
>     userspace to retry.
>=20
> So instead of normal wait path we get a busy loop on all secondary vCPUs
> before they get INIT signal. This seems to be undesirable, especially giv=
en
> that this happens even when Hyper-V extensions are not used.
>=20
> Generally, it seems to be pointless to mark an stimer as pending in
> stimer_pending_bitmap and arm KVM_REQ_HV_STIMER as the only thing
> kvm_hv_process_stimers() will do is clear the corresponding bit. We may
> just not mark disabled timers as pending instead.
>=20
> Fixes: f3b138c5d89a ("kvm/x86: Update SynIC timers on guest entry only")
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/kvm/hyperv.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)

Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>