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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: SN6PR12MB2767.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: 3f4a1dd7-1ee9-454b-dab5-08d8d42f6142 X-MS-Exchange-CrossTenant-originalarrivaltime: 18 Feb 2021 17:05:23.2188 (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: NUV7wMCEF6Glw9PmadvEg5JTnC21ZV9vAan+Xsy603L2K+MzPsYckhOueuM/yM34IgVdwebTs9ugjaPgpXII8Q== X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA0PR12MB4415 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org [AMD Public Use] -----Original Message----- From: Sean Christopherson =20 Sent: Thursday, February 18, 2021 10:39 AM To: Kalra, Ashish Cc: pbonzini@redhat.com; tglx@linutronix.de; mingo@redhat.com; hpa@zytor.co= m; rkrcmar@redhat.com; joro@8bytes.org; bp@suse.de; Lendacky, Thomas ; x86@kernel.org; kvm@vger.kernel.org; linux-kernel@vger= .kernel.org; srutherford@google.com; venu.busireddy@oracle.com; Singh, Brij= esh Subject: Re: [PATCH v10 10/16] KVM: x86: Introduce KVM_GET_SHARED_PAGES_LIS= T ioctl On Thu, Feb 18, 2021, Kalra, Ashish wrote: > From: Sean Christopherson >=20 > On Wed, Feb 17, 2021, Kalra, Ashish wrote: > >> From: Sean Christopherson On Thu, Feb 04, 2021,=20 > >> Ashish Kalra wrote: > >> > From: Brijesh Singh > >> >=20 > >> > The ioctl is used to retrieve a guest's shared pages list. > >>=20 > >> >What's the performance hit to boot time if KVM_HC_PAGE_ENC_STATUS=20 > >> >is passed through to userspace? That way, userspace could manage=20 > >> >the set of pages >in whatever data structure they want, and these get= /set ioctls go away. > >>=20 > >> What is the advantage of passing KVM_HC_PAGE_ENC_STATUS through to=20 > >> user-space ? > >>=20 > >> As such it is just a simple interface to get the shared page list=20 > >> via the get/set ioctl's. simply an array is passed to these ioctl=20 > >> to get/set the shared pages list. >>=20 >> > It eliminates any probability of the kernel choosing the wrong data=20 >> > structure, and it's two fewer ioctls to maintain and test. >>=20 >> The set shared pages list ioctl cannot be avoided as it needs to be=20 >> issued to setup the shared pages list on the migrated VM, it cannot be=20 >> achieved by passing KVM_HC_PAGE_ENC_STATUS through to user-space. >Why's that? AIUI, KVM doesn't do anything with the list other than pass i= t back to userspace. Assuming that's the case, userspace can just hold ont= o the list >for the next migration. KVM does use it as part of the SEV DBG_DECTYPT API, within sev_dbg_decrypt(= ) to check if the guest page(s) are encrypted or not, and accordingly use it to decide whether to decrypt the guest page(s) and r= eturn that back to user-space or just return it as it is. Thanks, Ashish