Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <117fd526-a241-2f01-47b5-e40e1803124b@suse.com>
Date:   Thu, 23 Jun 2022 11:51:42 +0200
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
 Thunderbird/91.10.0
Subject: Re: [PATCH v2 1/3] x86/xen: use clear_bss() for Xen PV guests
Content-Language: en-US
To:     Juergen Gross <jgross@suse.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        xen-devel@lists.xenproject.org, x86@kernel.org,
        linux-kernel@vger.kernel.org
References: <20220623094608.7294-1-jgross@suse.com>
 <20220623094608.7294-2-jgross@suse.com>
From:   Jan Beulich <jbeulich@suse.com>
In-Reply-To: <20220623094608.7294-2-jgross@suse.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?bzZNMndKRjZMZGllVk8yS3hqV0VEYytPUi9lalJESDhIY1V5RHBkdlh2V05a?=
 =?utf-8?B?WlhsT0VpN3ExenJqYm9TUnhjTWszVENWY1JDZ0FvRUNhM3F2dlMwMldqdzdI?=
 =?utf-8?B?R09UTFMrNWJ1QmhIVzkvMXZxTkkwV2JOUFd2aFB0UlFRaXE3aGJZZVVJNTlu?=
 =?utf-8?B?QTVGSkFGWTdsV29ZVFBpR0o3VnF1QTk5bXZ3a0NmUjk4WmkxK0lnUSs2UWgw?=
 =?utf-8?B?T01OTlVMeEY2eWhkTSt0ZjFHRUpEMGpSNlRRTm1wTUoxOE9Zd0xTTVBYakNQ?=
 =?utf-8?B?QXczRm5Vc1BERFMrTXlFbHhoQ0d4ekEzeUpoVDJOSE51Ym5sSjlpdTh4L1Nt?=
 =?utf-8?B?aFhzUVFnQk1NZElkSGtISkRXV1p3Rmkxdk92RGRHZE5FZEpoakVrd2JjL0tI?=
 =?utf-8?B?Z050UFZCM1lHUjduSiswNVVwUEVoTWJvOE8vZTBtOGVpeEtoRXk1bDJTaS9t?=
 =?utf-8?B?OTRTSjdUbHdlT3dZOFlTVG44WkdBTExHSm43dlJNODYrU0lvaERIdm1XcnE2?=
 =?utf-8?B?N2JQekFBTjJaT0NCUHlxbFRpOVlmeGMzMDMwZFcrNUZIWHAwb0xZSGh6S2pM?=
 =?utf-8?B?T3BNUHZlZmlwcyt2NU5CSmpiZEM0VTJiRWp5S3RsanI4Qm9ldldSblQxSXdQ?=
 =?utf-8?B?TFlGenlwSHlYRDdVNDAyU290bG1nUWZ2Rm5KZ2M4VmI4WjdlRDNaREI1UWMy?=
 =?utf-8?B?SG5jVUZsc0ZBd3hRTWFVNklKMmtoVklkZkRjTHBJUG1XMEdaNWlpY0hZcUNJ?=
 =?utf-8?B?eVFtM3N1Z0NITmhxVHBXQ2wwM1BENWpxWm5zQXdEOFNLQUV1VTRpMWJGUVR3?=
 =?utf-8?B?RWdQd3FJNndKVEJvZ2NZUmZpbUxINE1oY0N3MGp0M251RUFySjZISi8rWi94?=
 =?utf-8?B?UG0xd0VJMzI5ejVvK2FhNjFEYUJZeTNnUFN5MHF0cXppYXVzcjdubktnRGJt?=
 =?utf-8?B?S0JrZS8ybDVaVDVvMXp4a1M4VWJWMjRmaXhTdTVnTzdLand3ak1wRjlCMnha?=
 =?utf-8?B?QWt0MlExYWFWMGd5M0FFck1Sa2N4ajYwTjUzNHdPUmVvREwrVmNaNmFseHJ5?=
 =?utf-8?B?S0NJYi9KUFlJVU5mZWwySURVSHJvVENQSFhya25WV0VLK2swekx0U1pFcEV5?=
 =?utf-8?B?NC9vVU5seWJTSVVMWkJJcWpCa005cDRKZU1aRjZTVE5BTjMxS2NMNUYrNHVZ?=
 =?utf-8?B?OGRkQ3dGek1BUEYwYk9zMU1aN0J2WVQ5b2VHdkg0NUY4Y2ZYenJBSlZJbzM1?=
 =?utf-8?B?TlNRK3pYUmNvWDZpVnZKNUpZQjNoRnVkSUVQd2ZnU1FBcE9ranZ1NVVCL1px?=
 =?utf-8?B?SjhwR1UzMTBVNEN4UnlzMnRZSVBHYmhSV0hxa2xpV3ArYlBXRUhkbmdQZ2JB?=
 =?utf-8?B?NUdlNzVXNmJhRVF2eXRRdWZTc254cjA5cFNkcm1vWkR6ekVvSFlybjdzWGZ2?=
 =?utf-8?B?c1JBRThxR1NjNTk1cG8vbW1lS1pncFVJU0JNQW9aOGM5bkw2cmlnZGVoT2I4?=
 =?utf-8?B?d04xTmwzb1RCcDdPSk1FRVhoYmZUNnkxYzQ0MitUMW5XZi96S2hhWGYyWC9a?=
 =?utf-8?B?S3gzdUt4cmhiVGh1NG9rVktjWmdwU29zdGhsbGJuVWxuTjVzOVRJQ0FIdjdq?=
 =?utf-8?B?aVhRK21BYU9wR05TNU9rNDd1NS9rMlFTRlk0M01yanA0SUhtbUo2RzdjVjRM?=
 =?utf-8?B?VGhKTkxlWUc1dTR6dUpyK0ZvMEJxbm16N25uSDM0QnU5b3R3RE43UWt2Zmkz?=
 =?utf-8?B?ZG9JaW5NQTJ1V1VNbnp2cDd4cmIzblJ5S3JLUjNyeUZlcFhQS0N4U3FNUUxs?=
 =?utf-8?B?c0lNQlJPcW05aThsL2pwMWxneHY3WXVIVWRFeUhHWCtFdFJvLzIrL0U2b3V5?=
 =?utf-8?B?akZOd2JFcmhuS3NBdXNvQW8rdndpRldFQ1FaV2dtSi9IYUxBQXgwdFZSSHkr?=
 =?utf-8?B?eGc3Zkx2bmc3RDQ2Nm1sZlRRZlQxK3NhVHJ0OXplbGIrRDllZ1BHRzlMWFZx?=
 =?utf-8?B?aXdRUmtVUzd5VEMzdi9xR01zRFNmbkNDamt1d0Y1MlZuR2hKemFPWFFmaW1C?=
 =?utf-8?B?ZklmZnRzU0NEMzBudm5KcnpvWDdTVUdib0R2T2FLWmdDMU5pMTdoeExKcUtv?=
 =?utf-8?Q?268NdWPJWZKXNEuR3qskNoJ+b?=
X-OriginatorOrg: suse.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 4e8848dc-1266-4755-f88c-08da54fdfb9b
X-MS-Exchange-CrossTenant-AuthSource: VE1PR04MB6560.eurprd04.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 23 Jun 2022 09:51:45.2670
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: f7a17af6-1c5c-4a36-aa8b-f5be247aa4ba
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: bfyGMPdbQf6yUA21JLyChEz8CUhNOkUHAdwAj+tMT8bbiLPYqpg/a/YCfUHQMWMIIFca3xzuLDMNdgF2DQuT4w==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: AM0PR04MB6324
X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIM_SIGNED,
        DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,NICE_REPLY_A,RCVD_IN_DNSWL_NONE,
        RCVD_IN_MSPIKE_H2,SPF_HELO_PASS,SPF_PASS,T_SCC_BODY_TEXT_LINE
        autolearn=ham autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
        lindbergh.monkeyblade.net
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 23.06.2022 11:46, Juergen Gross wrote:
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -1183,15 +1183,19 @@ static void __init xen_domu_set_legacy_features(void)
>  extern void early_xen_iret_patch(void);
>  
>  /* First C function to be called on Xen boot */
> -asmlinkage __visible void __init xen_start_kernel(void)
> +asmlinkage __visible void __init xen_start_kernel(struct start_info *si)
>  {
>  	struct physdev_set_iopl set_iopl;
>  	unsigned long initrd_start = 0;
>  	int rc;
>  
> -	if (!xen_start_info)
> +	if (!si)
>  		return;
>  
> +	clear_bss();

As per subsequent observation, this shouldn't really be needed: The
hypervisor (or tool stack for DomU-s) already does so. While I guess
we want to keep it to be on the safe side, maybe worth a comment?

Jan