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: <1653609a-42af-b7cd-9d30-fb2bd5721080@suse.com>
Date:   Tue, 10 Jan 2023 10:38:58 +0100
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101
 Thunderbird/102.6.1
Subject: Re: Problem with pat_enable() and commit 72cbc8f04fe2
Content-Language: en-US
To:     Juergen Gross <jgross@suse.com>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
        "Michael Kelley (LINUX)" <mikelley@microsoft.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>
References: <BYAPR21MB16883ABC186566BD4D2A1451D7FE9@BYAPR21MB1688.namprd21.prod.outlook.com>
 <03edcbc5-2dd7-1ddb-bafe-8412d8fc95aa@suse.com>
 <ba24157d-92fc-f472-9ef5-4eae3c63c12e@suse.com>
From:   Jan Beulich <jbeulich@suse.com>
In-Reply-To: <ba24157d-92fc-f472-9ef5-4eae3c63c12e@suse.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?YzU2ZFd2S2l6TmdlQkw0a2JONkppY3dQVTBCZzNQV212SFI5UlU3TjhCYnFR?=
 =?utf-8?B?QlFTVmNtZzNIWnlVR0J3UU5JazdwSnZFaEtGcTlsZnRxK3A2RkZFeThDc2Zq?=
 =?utf-8?B?Z05LMHZRZHZ6QVQ1M21nTXdYeVZkUWd3SC9lTFdpNXZraVV5bW05R1praFhT?=
 =?utf-8?B?aVZGbjhUNmxVbERudHVSK003L3pTTDZIeGdTU2VIVmp4OS9TaUZQRlV6Y1dW?=
 =?utf-8?B?RXk4NSt0Yzkwb0VkalJMSGtEQWZIWXlKY1JTb2ZhcHRBemJhOURpbGgrQ0x1?=
 =?utf-8?B?aDNTZllDSjE2M0Flbk9zUUF4d0Y0ak5nd2NjM3p0a1hEdlpWT09NT2RXeFYw?=
 =?utf-8?B?NVExQjNCRmJwQUpNcHJrck93SHY2UitsZU1aWWFxRjB1Z3Z2MnRLTkduYVk3?=
 =?utf-8?B?VW1hUktJWGxMZURKYWFWRHdnRWdWbWZZNjVVZytYS3hEVEZzdENLbTFYR1Ex?=
 =?utf-8?B?QnN1NHJqMG4rRWh4Q0k2bkFRWm1UZmxIZlk5VFBWZXZYSTdGUlp6aEg2cjg2?=
 =?utf-8?B?Q21jNkxGRG90UXpqelZiOVhSVVcyL1NTYWhNRk14N1hYY1RxWjFKdHQ2LzFk?=
 =?utf-8?B?bjdrREZWTTA2MmVlOUNGcTBnQ2tXeG1heVpKTzl3dkpmaDViTXhOVE14TFNG?=
 =?utf-8?B?cTlZL29MQmNSTS9jRTZZZnBXbjlRNEdRdjhJWkk5RVY1bDdLWlkrc1NrK0Rr?=
 =?utf-8?B?NG4xbUw3aTB2L2s5cWlRMm9LcGZCUDU4UWVxL0Y4NXgyS0IycEtMaFpjdDZD?=
 =?utf-8?B?Wlpib2poWWJvKzFMK2FLeHpHYkJLTDV2WmVTdWRFWndzQWF3RWtlekEyVnl5?=
 =?utf-8?B?WDJOWURLNm9Gb0JKU3dkYVBYRXczamMxNXNVV1Z4eE9VU3YxcGRDY3M5MlJK?=
 =?utf-8?B?U2FzOFpUbXZCQXVwNElwdysyN3NXRjFLbmErUVdSdnNBQi9wUm45ZU5iQngz?=
 =?utf-8?B?eDRMSXR1S2p2cDRNaTFjRFNZVUhvT0ZhMWVaWXZ0RlZZMjQvdi9ablZ1cWI5?=
 =?utf-8?B?T09uVi9pMGJkUi9VU3BNVVJ5K2h2OFNNMUhiRkxpKzd4WG91dnpUenNvUUZi?=
 =?utf-8?B?dG9VcmladU5iVFRDTytuR1JwN25nWkdmQzRpSzRFTS8zWmhqTld5ZC9nY2ov?=
 =?utf-8?B?UDBMQW1mdWVUdXFRYVV1QmtPVUQ4TytIc2grRzRXRm5YWTU4UyttUnc0eHBH?=
 =?utf-8?B?UW1aZ3R2bWNKdnNtcGQ5a0J6STdVbnVLbHRsK3hla1BDZjFUSDdTQ3VsRSs0?=
 =?utf-8?B?Wk1YUHJwL3haVS9rbVBhZjJreURweit0Mm9kK3JFK0VQeldvNnhDSy9UV1JM?=
 =?utf-8?B?WExSQlNUcHdhZjA5KzAyKzhLOVZaQkJYSnVzbHA2RU5YU0RNaFRvK1ZhdHdO?=
 =?utf-8?B?VVp0dktFVzMzU2NHNFZ3bHNJNkdjMWx5SGJ3RDdWRWtPYnMrU0N3dlA5UVpi?=
 =?utf-8?B?TVpjd1ZVMHM3ajJtQkZ2c2JJbFI4TmQ3Wm1TSGRPV0lGZndpbnA1d0lRWXpX?=
 =?utf-8?B?ZVBwL29vbmNoMEpRZE9GcmRpeDNRMFJrWHNIVVFMU0wzQ05vZ0hEN0lSaEha?=
 =?utf-8?B?Y0JOekJrWWdwY1Y0NzNHSmhiTzdlbmphUXVNQ3lDQ0tqMXhYYm9lVDQ0QXVV?=
 =?utf-8?B?ZW9Pa0RQYXVaOVpER2tXNE5qeitBenIzRytoOWF1Mm9IZmNHcUdSeEQ2VVFS?=
 =?utf-8?B?c25iVEZWdEEwV1FlNExIODVVUUJlSkxQbUI4c3IwVUQwVTJ1WXlrUFU1N0t0?=
 =?utf-8?B?RHhJQkR3SW1wb2RIVjQweTFCOGJrMzFMcWFjS0J1elQwWVRtc0hURkRTaFZC?=
 =?utf-8?B?aG5MeU9SU215RjJVcWNyb0JJZWlpV21EY2ZyMEdIdnZmbmZucWhEbXhWdC90?=
 =?utf-8?B?dFJvdm16TFFtd2IrK1ZwZE5Qc1VqVTMxWW5MdDV3N0txZWVZMHJIeVVyNFZj?=
 =?utf-8?B?U0xtU3pSRjE2T3FFd0R4UjdQY0IyNnJONHFhV0FLMlZST3JmRkxpTTg5K2o3?=
 =?utf-8?B?NEpYRDU3NzJENGoySHdYZGJFK3RkT1Y0QlpFK2ZtOEo4ZGpuNkJBVUxnUWxk?=
 =?utf-8?B?Vm43Y2VKZlM4VklMdUdjSW16MUdoNnc2NC91Q2N2eklubm5TMGRxU0doWGNC?=
 =?utf-8?Q?D2kDcCAVBwI7eUHpP6LGiI7FW?=
X-OriginatorOrg: suse.com
X-MS-Exchange-CrossTenant-Network-Message-Id: 55782c39-384c-4f77-2050-08daf2ee8100
X-MS-Exchange-CrossTenant-AuthSource: VE1PR04MB6560.eurprd04.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 10 Jan 2023 09:39:01.2691
 (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: iMB3rXEu78pG8LBmIHS4jDVRaIiPhrtDa/I3UNm/tVSsJDkFEijEKrLZeJ3Ns1FjQadP39iWz2IN5Q3Bry4Uwg==
X-MS-Exchange-Transport-CrossTenantHeadersStamped: PR3PR04MB7386
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 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 10.01.2023 06:59, Juergen Gross wrote:
> On 10.01.23 06:47, Juergen Gross wrote:
>> On 09.01.23 19:28, Michael Kelley (LINUX) wrote:
>>> I've come across a case with a VM running on Hyper-V that doesn't get
>>> MTRRs, but the PAT is functional.  (This is a Confidential VM using
>>> AMD's SEV-SNP encryption technology with the vTOM option.)  In this
>>> case, the changes in commit 72cbc8f04fe2 ("x86/PAT: Have pat_enabled()
>>> properly reflect state when running on Xen") apply.   pat_enabled() returns
>>> "true", but the MTRRs are not enabled.
>>>
>>> But with this commit, there's a problem.  Consider memremap() on a RAM
>>> region, called with MEMREMAP_WB plus MEMREMAP_DEC as the 3rd
>>> argument. Because of the request for a decrypted mapping,
>>> arch_memremap_can_ram_remap() returns false, and a new mapping
>>> must be created, which is appropriate.
>>>
>>> The following call stack results:
>>>
>>>    memremap()
>>>    arch_memremap_wb()
>>>    ioremap_cache()
>>>    __ioremap_caller()
>>>    memtype_reserve()  <--- pcm is _PAGE_CACHE_MODE_WB
>>>    pat_x_mtrr_type()  <-- only called after commit 72cbc8f04fe2
>>>
>>> pat_x_mtrr_type() returns _PAGE_CACHE_MODE_UC_MINUS because
>>> mtrr_type_lookup() fails.  As a result, memremap() erroneously creates the
>>> new mapping as uncached.   This uncached mapping is causing a significant
>>> performance problem in certain Hyper-V Confidential VM configurations.
>>>
>>> Any thoughts on resolving this?  Should memtype_reserve() be checking
>>> both pat_enabled() *and* whether MTRRs are enabled before calling
>>> pat_x_mtrr_type()?  Or does that defeat the purpose of commit
>>> 72cbc8f04fe2 in the Xen environment?
>>
>> I think pat_x_mtrr_type() should return _PAGE_CACHE_MODE_UC_MINUS only if
>> mtrr_type_lookup() is not failing and is returning a mode other than WB.

I agree.

> Another idea would be to let the mtrr_type_lookup() stub in
> arch/x86/include/asm/mtrr.h return MTRR_TYPE_WRBACK, enabling to simplify
> pud_set_huge() and pmd_set_huge() by removing the check for MTRR_TYPE_INVALID.

But that has a risk of ending up misleading: When there are no MTRRs, there
simply is no default type (in the absence of inspecting other criteria).

Jan