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Mon, 2 Mar 2020 17:11:12 +0000 Subject: Re: [PATCH v2 2/2] KVM: SVM: Enable AVIC by default To: Paolo Bonzini , Wei Huang , Jim Mattson , Oliver Upton Cc: kvm list , Tom Lendacky References: <20200228085905.22495-1-oupton@google.com> <20200228085905.22495-2-oupton@google.com> <6487d313-dedb-1210-1c7a-160db2c816ad@amd.com> From: Wei Huang Message-ID: Date: Mon, 2 Mar 2020 11:11:08 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SN4PR0201CA0010.namprd02.prod.outlook.com (2603:10b6:803:2b::20) To MN2PR12MB3999.namprd12.prod.outlook.com (2603:10b6:208:158::27) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [10.236.30.248] (165.204.77.1) by SN4PR0201CA0010.namprd02.prod.outlook.com (2603:10b6:803:2b::20) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.2772.15 via Frontend Transport; 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X-Microsoft-Antispam-Message-Info: uYmbM5FKeQGgmGfN9HyNGZdtVnNMScK31f4YnY2VDeZfmoqTxrR1kOGye+82agnNyeU5TAc5yJJa2ayVPnOcnOjCGHVknAJPd+kxKSonfJKlD5IFfwXAeyXTYHy8yZvJ/eSzP/fDLhXvdCoREqMf6vuxQxkf7KzJYObLlz7TI0ECuO+dgwHyCBt8CyDM6DkH5TBuwGu9PZYnarblHMrwRBTd9gfhjI7r+g2GoHoURRAgJxJkVjgMxw5D0ZB06+jVe2WTq1v5rWktYBT8dtEHNGesiptJDU78Xi9igX4Ndi51hcxrEP15cx6rz6yV33BACFQUveB4oM71o+HSoBhpM8NOaQhBzuRODnGzza57IN9mYKAu2F0jIKpXSgIX6/F67xlUk9HjVlmWZaE5oERYqXAGn3mxzDCgus98xTN34DCxdOf59d8lObfcjiPZulJ6 X-MS-Exchange-AntiSpam-MessageData: FXtgRnezqqEQpOx+6Mq95KJDd0ZAV69jpwJD9w7vJnLC1gBCnLLFiX730D71imkXOnY3pJwNlya8Bjrj8dyOrqbQ4xoBjVuG8kqaOhFFFO/KleHwF8+O3L4qS1KNxojBG5lM8ExWLyyBQyoBH+nFDA== X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-Network-Message-Id: 233abd74-b3e3-4634-c447-08d7beccb545 X-MS-Exchange-CrossTenant-OriginalArrivalTime: 02 Mar 2020 17:11:12.2137 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: 0I5lI1YgK6o2fnPZblqpyiML2TTrhh2nRkFXgoubb0zdJuSAgor5yZcugq7qp4H7 X-MS-Exchange-Transport-CrossTenantHeadersStamped: MN2PR12MB3007 Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 3/2/20 10:19 AM, Paolo Bonzini wrote: > On 28/02/20 23:47, Wei Huang wrote: >>> How extensively has this been tested? Why hasn't someone from AMD >>> suggested this change? >> >> I personally don't suggest enable AVIC by default. There are cases of >> slow AVIC doorbell delivery, due to delivery path and contention under a >> large number of guest cores. > > To clarify, this is a hardware issue, right? > > Note that in practice this patch series wouldn't change much, because > x2apic is enabled by default by userspace (it has better performance > than memory-mapped APIC) and patch 1 in turn inhibits APICv if the guest > has the X2APIC CPUID bit set. QEMU will work fine with this option ON due to x2APIC, just like what you said. If you feel other emulators using KVM will behave similarly, I can revoke my concern. > > Paolo >