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: <b73fd351-7b1e-564e-28ca-0eeb68e52773@suse.com>
Date:   Wed, 4 May 2022 11:51:17 +0200
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
 Thunderbird/91.8.1
Subject: Re: [PATCH 2/2] x86/pat: add functions to query specific cache mode
 availability
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>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Jani Nikula <jani.nikula@linux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@linux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@intel.com>,
        Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        xen-devel@lists.xenproject.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, intel-gfx@lists.freedesktop.org,
        dri-devel@lists.freedesktop.org
References: <20220503132207.17234-1-jgross@suse.com>
 <20220503132207.17234-3-jgross@suse.com>
 <1d86d8ff-6878-5488-e8c4-cbe8a5e8f624@suse.com>
 <0dcb05d0-108f-6252-e768-f75b393a7f5c@suse.com>
From:   Jan Beulich <jbeulich@suse.com>
In-Reply-To: <0dcb05d0-108f-6252-e768-f75b393a7f5c@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?TVNQMVhDNENaT2UzY0xSUkI5S2hvM1MxcGhWbmhrNFRGSlZjYnF5UEkrNjFM?=
 =?utf-8?B?NkJtT0dyR0NyRXFGdjhhWG5KR0pxaVpoQmpTVUE0ZlZnOHZ3RmV4aGdreDBW?=
 =?utf-8?B?SHhqMDhsNWNjWExyWDJvbVQwalI2dXFTNUQzOG9Ca21lRGQ3SmlWekJ5cFh3?=
 =?utf-8?B?NEpKU1JkU3dJYjE5eWNOczU4eDF0Rnh4Ly9JcDBScHhVclZuV0lzMDlqRVYx?=
 =?utf-8?B?RU50Y3FEL29UL2U2dHNGNU5FeTZPM3FtWXdXb1ZxUG51dWcwbEFqQksxWCti?=
 =?utf-8?B?aVV2eHZMdStmQUp2UitWbGlEbFNmM3hDaWFzOHphdXl4RURVYXd2T1g2bXZE?=
 =?utf-8?B?QVJDQzcxUnRzdkVtWkF5R1cxY1ZvbnQrRDQyVFlzOGxWMGc1ci9xTnFBbzJP?=
 =?utf-8?B?MytTOFU2RG9FSVdlbklWYi9va1hRckxSZUMrM292eVMzalZ1NXB5VXhpd0xx?=
 =?utf-8?B?S0srMUQrTFpFK3Y1dUQ3UExzcHkwYm92aDltMnRLVlhxZG1GRW9mZjY3amVj?=
 =?utf-8?B?ODVZZ2lyOFd6eHF2TFFmRHdhVVNKRFRXWGRacW9hYWM4bGlNRHFjRzBaTWFm?=
 =?utf-8?B?Vkk0UEpzaVcrK2M2RksyYjJQaWJIdnFWbVMzQnpuaElPdk1zN1R1dCtzSGVR?=
 =?utf-8?B?cHFuYWpJaEt5RnBmWmU4TXBCelhROU5MeXlsMk1xUGVUTmcyeHIweGFOcm9I?=
 =?utf-8?B?TEhwa3VZNXdjZlNEbDgyYjh6SjV3RStXam9vakw0MEN5QTUwc1V4R25ib0dS?=
 =?utf-8?B?R3BLTk95bHVrODBEcjdKRHAwL3p6dkJUQUpwUVpnTnNuL05tbTZUbG1PUXFv?=
 =?utf-8?B?MDNhNlVXU2VuWEZDYUVSczZlYnZIVEhTVUlMZVMwV2ZrSnlTT1dKaWFsMWVr?=
 =?utf-8?B?MUJ2TFozLzI2M2MxdWx3SU5tdlcwVGl3V1B4Wk1iVUhLMUgwYkZ2aTZGdWl0?=
 =?utf-8?B?LzNtYVV3UE42VVpPM1JuSFJNRzBLMm9ubUNub25sbVRGMTZGTE1OL0tKeXo2?=
 =?utf-8?B?Mzd0TFlhbmhqZnA3emtOK2lvV0trNklrNnpDa1M5Z3VjSUlFMUxWaHY3OXVV?=
 =?utf-8?B?b2ozNlgwV0JDa0tqNnFKL1k2SXp0SlBwdDdHWU5FbVlIMTFVQ2k5QlBDOHBJ?=
 =?utf-8?B?bUpiUkkwUURQUWhBRzJET2o3elRJYnZ5TFRQZkRKbVJCVExEWm9HanRRWUpF?=
 =?utf-8?B?dXB5U0ZVLzFJbWtqVTNPbkpkUlVlc1NBOXFDVFVZUHdnWVlrc3lJQkF1VzBj?=
 =?utf-8?B?MHlteTJ4YVArQ1JvcFdzekhPK1RyTEhFbUhXSy82RFdrdS8yVFJ2WGgyOFBv?=
 =?utf-8?B?SXBZUUYxS2xlS1U2Z2NDbXk1ZG9oTlBIRFJkMDA4eUNKUW1ibVZBcFNJM2tP?=
 =?utf-8?B?NUVOSmdrOWpIVlZhRXBrS0FTRXAzZlUwaDBqZDR2U3JTL3JLM1ZLUE5LcnZC?=
 =?utf-8?B?NXEvWUthdW03c0d6VXpSWWtFdlRpZG5jRy96bGJ5bTRSM3hVMm1iTlBvZmdB?=
 =?utf-8?B?cWk1elNHNkMzN3l1MVZ6cGlNUmY1OStPelZuNzdVY3ZaaGFJZjFuMFVpY0xs?=
 =?utf-8?B?elhOeFdDdXBobHVRWDRJYWJKSmh0aVZIV3BlUFQ4aS9qeTJTTkhjMzZ6cHpT?=
 =?utf-8?B?dnQ4bFp0U0VkNjBZbGJqcm43SmNud08xRmtNOXhPUjRrd0czVVJodUlPVU5F?=
 =?utf-8?B?c2dYRUJHZi9JSGMvK1o1YU1kaE1UWkN3SThqRWdEU2R4a0c2MkQ4T1MwZGtD?=
 =?utf-8?B?UUY1bllxV3BrSGxIQ3lWSEk1OGtNdXVwTlNkUzRwNUVjd1dMYXFLWk5EZWJv?=
 =?utf-8?B?Z0JqNjhCeXAxL0s3R1J4ekFJQmljTk9TL2JsazMzR0NXbGtLN1lxRDVBaVJO?=
 =?utf-8?B?dXUvY3Z4ekxIaXdZb2VsdEVsKzh2Q2lMSHVUU0NxRGU2QVhVMnU3aklRM3NE?=
 =?utf-8?B?dGxxQVhjNTdIQXBHTk4rTDRTT2RUY1VzSVNUblB2L1EvWGo4VVJmbVp1V1ZY?=
 =?utf-8?B?QlIvcmNSMDBTS1lsVVB6M3c0dTAzeTM3cWVZNW5zU2xrRVhKc2xGSmxqd1lN?=
 =?utf-8?B?QStvYmJrUHYybEFta3YxZE8xQ1lYQmJiM2ZSNGUrczMzLzE4QWJmWTlJa3Bq?=
 =?utf-8?B?cEdVZUZVTHBLYndhcS9XRkhsK1pqbEg1RFg5K2ZJSHVjSkR3UHNhb3N6T2JU?=
 =?utf-8?B?TVN4R0RLWmpxRUJzL2ZSMFBlbHZDVTdVRGFwNm5wL2dua01vUU1NeXVqUXhG?=
 =?utf-8?B?eFJYMDRCaXZuQzFPSVNsWUVXeXBMVWlNYmNXbnlJT29jT1dVUlZiY2txS3hN?=
 =?utf-8?B?TGdXNS96ZkJWemVQTXp4TlFEWmI3MXhYN1c4ZUxlbDBYTEs0bVNtQT09?=
X-OriginatorOrg: suse.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 6498b1c5-090c-4e8d-8a46-08da2db3a430
X-MS-Exchange-CrossTenant-AuthSource: DU2PR04MB8616.eurprd04.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 04 May 2022 09:51:20.4324
 (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: x18Q+h6dqK4TJ6XqYgwEsWrAjakGvYPTmq6w0o4XhpedTHIzVkbHqjeHKAcS43UmADypyJ6586HslpIWsanLHA==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DBBPR04MB8043
X-Spam-Status: No, score=-5.3 required=5.0 tests=BAYES_00,DKIM_SIGNED,
        DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,NICE_REPLY_A,RCVD_IN_DNSWL_LOW,
        SPF_HELO_NONE,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 04.05.2022 11:14, Juergen Gross wrote:
> On 04.05.22 10:31, Jan Beulich wrote:
>> On 03.05.2022 15:22, Juergen Gross wrote:
>>> Some drivers are using pat_enabled() in order to test availability of
>>> special caching modes (WC and UC-). This will lead to false negatives
>>> in case the system was booted e.g. with the "nopat" variant and the
>>> BIOS did setup the PAT MSR supporting the queried mode, or if the
>>> system is running as a Xen PV guest.
>>
>> While, as per my earlier patch, I agree with the Xen PV case, I'm not
>> convinced "nopat" is supposed to honor firmware-provided settings. In
>> fact in my patch I did arrange for "nopat" to also take effect under
>> Xen PV.
> 
> Depends on what the wanted semantics for "nopat" are.
> 
> Right now "nopat" will result in the PAT MSR left unchanged and the
> cache mode translation tables be initialized accordingly.
> 
> So does "nopat" mean that the PAT MSR shouldn't be changed, or that
> PAGE_BIT_PAT will never be set?

According to the documentation for the option ("Disable PAT (page
attribute table extension of pagetables) support") I'd say the latter.

Jan