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: <9be1413b-ce30-acf8-86fb-3e302dc98396@intel.com>
Date:   Thu, 13 Oct 2022 10:33:29 -0700
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101
 Thunderbird/91.13.1
Subject: Re: [PATCH] x86/fpu: Remove dynamic features from xcomp_bv for
 init_fpstate
Content-Language: en-CA
To:     Dave Hansen <dave.hansen@intel.com>,
        "Yao, Yuan" <yuan.yao@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     "x86@kernel.org" <x86@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
References: <20221011222425.866137-1-dave.hansen@linux.intel.com>
 <BYAPR11MB3717EDEF2351C958F2C86EED95259@BYAPR11MB3717.namprd11.prod.outlook.com>
 <9b198ed3-4b2d-c857-710b-3f7115bbcf74@intel.com>
 <78f2021e-339f-9dfb-2aee-51f58ea77ec9@intel.com>
From:   "Chang S. Bae" <chang.seok.bae@intel.com>
In-Reply-To: <78f2021e-339f-9dfb-2aee-51f58ea77ec9@intel.com>
Content-Type: text/plain; charset="UTF-8"; format=flowed
Content-Transfer-Encoding: 8bit
MIME-Version: 1.0
X-MS-Exchange-AntiSpam-MessageData-ChunkCount: 1
X-MS-Exchange-AntiSpam-MessageData-0: =?utf-8?B?QnRydkZpQmp3U2JHR3Q2Ums5VHgxMkNzbTNyUlZOZ3RrZjVjdzRML0tadlFz?=
 =?utf-8?B?aDgvSHdNTzc5Rk13U2Z0MzljaksxZUFUVTl2c1cvMC94dXdhS254cDJ0Tlpy?=
 =?utf-8?B?VWFDdjBHWE9sRXBlVytRTm9BZzBjM3VaV1QzZXFKWkZSSEpOMGJqY29YSjYv?=
 =?utf-8?B?cy9zNDRhemt5Wk55V1BsV3Mxcm93NlJWOXZmTm5LNEEyaUc2Y2l6d2VRUDVK?=
 =?utf-8?B?Z09NYzEvcDhubFJyeWdJaW5raHlQTEx6VVRkUmdULzJPOUJ6c29VVm5UOCsw?=
 =?utf-8?B?dTR0NU16M2t1Lzk3TmJnL0tjanlOYUc0OEtyNC9maHYvdGxNaTZYb1NpakJ4?=
 =?utf-8?B?QzJ3dGF1VGFhWU90Wk1aOXpwUkRURmpaUXpEblVFT2dXSW0xcjZlQml2SE1k?=
 =?utf-8?B?RXBCeW01NDZRYnk1V3NBaitYSW50bWVtQU5wcnJVYjVBSUJ4VlcveTRIK1Yz?=
 =?utf-8?B?UWk0SjhyQUFPaFJmVCtudnhVNHYyZFU3NTU2WTY3U3drby92ZHZnczJ1RzJk?=
 =?utf-8?B?VENzY1FPbHVmSzR2andPZUtZM2ZtQkttMXJ5L1VZK1RlZTlXV253MGtWZUZX?=
 =?utf-8?B?R3U1ZnJIWDhHa2JuT3FnSFNaYVZmM0JGa1UvY2FRNUpuZWxONitzTlFZcWFw?=
 =?utf-8?B?dlcxUzFJK0dZSkZUT1M3RnQxZ2RTQ0tHcjF5QytMV25qcEN6d0hHcWJHb2hH?=
 =?utf-8?B?ak94NFhxV2xjcTA5RWtrb084eURBRmhOdlpmMjlkQXZOWUExeFFpK3haY2Ro?=
 =?utf-8?B?c1cxZXVXOVZzSW5Fbjdnc0ZFM0pGRVU0NlcvVURXQ2RCN3hqWlkvOFVFRXda?=
 =?utf-8?B?Yk5qTVNRc1ZOSXdRLy8zeGxPRHJvVjFYSVgydEpMZ1NIWHdrTjZLNit4Q0hN?=
 =?utf-8?B?QW1tTkxEQm13S09rNjZnMTJpV1IxSlRad0xiNDBlOWVyTFYrTUt1azNwejd5?=
 =?utf-8?B?Ris4WHBGTUxSckxHSjNkNmFPZm5Ic0FHNGdaeW8zOUhvaXgrRHJLV0pXRTRw?=
 =?utf-8?B?WTBZTHhzVEJiRFh5eEhFYTRHa2VCcHhtMkhZL3VVWGZoY1c4MG9LWTlhbERO?=
 =?utf-8?B?Smt2RjRWc1B4dlhpaER6Z3VkR3hNRjJBRG9XOEZJeHphTFpCSlNiQTVzc2I3?=
 =?utf-8?B?VFNwcGlITHBwZGpSSzBqMWNhQXVOa2dHS3FMem42SVNpTy9ER3pQSnVNK0Rs?=
 =?utf-8?B?NmEraDJKVmcrRlhMYktkY1FSbmpmelljS1RkMzArTGJ4YXQ1aURMQ1lFRWRx?=
 =?utf-8?B?ZVhoN21ZQytkZHByb09XTlNCNXMxZXFuOFB6Nk1rUDhhcW55NHlJalppMHFO?=
 =?utf-8?B?dlp2QmpWUHd6d2tYZU02NnZwWlM4aThLN3JDZmg3VDJZaW9zTGFZSGFKWHpK?=
 =?utf-8?B?Ui83OWN4U1gvUXBCcDgrOFVXYzBwSlkyb05XbFZHUFIzY1FEcVgrWWpsMjE2?=
 =?utf-8?B?aEpWTzhMcGZUZ2E2RmVISGd4dDI0UWhFTXBWN25WR2NJTUpYbVNybWFVM3RP?=
 =?utf-8?B?ZXdZS3N0Y1VoQUQ4VkZYY3JpNGYyYzAwNWx1UXhCclB5ZTc0WVJhajVRSm52?=
 =?utf-8?B?NEV0TmFQR3lpRUV2RXVEaDYrY1JtYVlMejJQUHpuN1R3cGtVR2tzSllhT2lk?=
 =?utf-8?B?akdxQzVLTVJBd0dOUTBzNjJOdFNya3ZMY2dBZXkxdHNKVURhOW1URVdFbmdv?=
 =?utf-8?B?SFBFNFZtOGVxc0dNbW8vTnp2UXFhYjRkbXNhb0Z5cHYvVGM5TE5BRkJYTHJW?=
 =?utf-8?B?ZmxaSmpnZFlpcDUyQkNGNWlLczNwWWlBUVVXdjd0d1UxNS9qWU54QjBVTVk4?=
 =?utf-8?B?S0l1dkhEYzRnUm54QjA2YU5xeDVQd09aNlhzZUIyTGlUcEQ1eTB0ajA4MmZv?=
 =?utf-8?B?bXFlbnlYaVd0ZnF5U3dqTnFFRXJJenFVZjV0UFlUSS9RbFZHTWp0Z2lXSUtZ?=
 =?utf-8?B?U1NwMEZJNklxQ3JLdm1xL3VWSFlwT3RzamRSVEFXMk1zNHdTVXBFdGM1MDhs?=
 =?utf-8?B?cXc1V3ZIWDk0NncxOWo0UDZ2Z1NsRGJOK0ZKL25uRU5MQ3pFUytwb0haaWY4?=
 =?utf-8?B?ems1Rm8zaTcwQUZCbzdKU3JJc2lnRWU5SCt6eFc2UWlVL2RHSzFoTzB5ODRt?=
 =?utf-8?B?VXBIVW5YSXQ5cUdyckdmYndLSTFsaGtqb1pPdTgvSUJUMlpHY1FDRVg0ZHdO?=
 =?utf-8?B?RWc9PQ==?=
X-MS-Exchange-CrossTenant-Network-Message-Id: d5d71074-2051-44fe-449a-08daad410bc4
X-MS-Exchange-CrossTenant-AuthSource: PH0PR11MB4855.namprd11.prod.outlook.com
X-MS-Exchange-CrossTenant-AuthAs: Internal
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 13 Oct 2022 17:33:30.9643
 (UTC)
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 46c98d88-e344-4ed4-8496-4ed7712e255d
X-MS-Exchange-CrossTenant-MailboxType: HOSTED
X-MS-Exchange-CrossTenant-UserPrincipalName: 50x1laLWZBr1N8UCstp79/KhSRzj82/IpmI13WqLcFtWvtDlSx1KMETjwk0sk95tR+UYFHYj/7bDmotgqY1nkHpMTQ1WmwLcJRWZbbKXalI=
X-MS-Exchange-Transport-CrossTenantHeadersStamped: BN9PR11MB5225
X-OriginatorOrg: intel.com
X-Spam-Status: No, score=-5.6 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,
        DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,NICE_REPLY_A,
        RCVD_IN_DNSWL_MED,SPF_HELO_NONE,SPF_NONE 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 10/13/2022 10:21 AM, Dave Hansen wrote:
> On 10/13/22 09:23, Chang S. Bae wrote:
>>
>> --- a/arch/x86/kernel/fpu/xstate.c
>> +++ b/arch/x86/kernel/fpu/xstate.c
>> @@ -1127,8 +1127,12 @@ void __copy_xstate_to_uabi_buf(struct membuf to,
>> struct fpstate *fpstate,
>>           * non-compacted format disabled features still occupy state space,
>>           * but there is no state to copy from in the compacted
>>           * init_fpstate. The gap tracking will zero these states.
>> +        *
>> +        * In the case of guest fpstate, this user_xfeatures does not
>> +        * dynamically reflect the capacity of the XSAVE buffer but
>> +        * xfeatures does. So AND them together.
>>           */
>> -       mask = fpstate->user_xfeatures;
>> +       mask = fpstate->user_xfeatures & fpstate->xfeatures;
> 
> I'm not sure this is quite right either.
> 
> Doesn't kvm expect that all of the ->user_xfeatures will end up being
> copied out?  We surely can't copy them from 'fpstate' if the feature
> isn't there, but we can't skip them entirely, can we?

No, we can't skip them. IIUC, the code will zero out:

	/*
	 * ... The gap tracking will zero these states.
	 */
	mask = fpstate->user_xfeatures;

	for_each_extended_xfeature(i, mask) {
		/*
		 * If there was a feature or alignment gap, zero the space
		 * in the destination buffer.
		 */
		if (zerofrom < xstate_offsets[i])
			membuf_zero(&to, xstate_offsets[i] - zerofrom);

		<snip>

		/*
		 * Keep track of the last copied state in the non-compacted
		 * target buffer for gap zeroing.
		 */
		zerofrom = xstate_offsets[i] + xstate_sizes[i];
	}

out:
	if (to.left)
		membuf_zero(&to, to.left);

Thanks,
Chang