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Wed, 2 Jul 2025 11:26:59 +0000 Date: Wed, 2 Jul 2025 19:24:20 +0800 From: Yan Zhao To: Ackerley Tng CC: Vishal Annapurve , "Edgecombe, Rick P" , "quic_eberman@quicinc.com" , "Li, Xiaoyao" , "Shutemov, Kirill" , "Hansen, Dave" , "david@redhat.com" , "thomas.lendacky@amd.com" , "vbabka@suse.cz" , "tabba@google.com" , "Du, Fan" , "michael.roth@amd.com" , "seanjc@google.com" , "binbin.wu@linux.intel.com" , "Peng, Chao P" , "kvm@vger.kernel.org" , "Yamahata, Isaku" , "linux-kernel@vger.kernel.org" , "Weiny, Ira" , "pbonzini@redhat.com" , "Li, Zhiquan1" , "jroedel@suse.de" , "Miao, Jun" , "pgonda@google.com" , "x86@kernel.org" Subject: Re: [RFC PATCH 08/21] KVM: TDX: Increase/decrease folio ref for huge pages Message-ID: Reply-To: Yan Zhao References: <4cbdfd3128a6dcc67df41b47336a4479a07bf1bd.camel@intel.com> Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-ClientProxiedBy: SG2PR02CA0029.apcprd02.prod.outlook.com (2603:1096:3:18::17) To DS7PR11MB5966.namprd11.prod.outlook.com (2603:10b6:8:71::6) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: DS7PR11MB5966:EE_|SA2PR11MB5068:EE_ X-MS-Office365-Filtering-Correlation-Id: 58d68f75-4d93-4d72-6f7c-08ddb95b5bda X-LD-Processed: 46c98d88-e344-4ed4-8496-4ed7712e255d,ExtAddr X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|376014|7416014|366016; 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Have the arch/x86 side of TDX grow > >> > > > > a > >> > > > > new tdx_buggy_shutdown(). Have it do an all-cpu IPI to kick CPUs out of > >> > > > > SEAMMODE, wbivnd, and set a "no more seamcalls" bool. Then any SEAMCALLs > >> > > > > after > >> > > > > that will return a TDX_BUGGY_SHUTDOWN error, or similar. All TDs in the > >> > > > > system > >> > > > > die. Zap/cleanup paths return success in the buggy shutdown case. > >> > > > > > >> > > > > >> > > > Do you mean that on unmap/split failure: > >> > > > >> > > Maybe Yan can clarify here. I thought the HWpoison scenario was about TDX module > >> > My thinking is to set HWPoison to private pages whenever KVM_BUG_ON() was hit in > >> > TDX. i.e., when the page is still mapped in S-EPT but the TD is bugged on and > >> > about to tear down. > >> > > >> > So, it could be due to KVM or TDX module bugs, which retries can't help. > >> > > >> > > bugs. Not TDX busy errors, demote failures, etc. If there are "normal" failures, > >> > > like the ones that can be fixed with retries, then I think HWPoison is not a > >> > > good option though. > >> > > > >> > > > there is a way to make 100% > >> > > > sure all memory becomes re-usable by the rest of the host, using > >> > > > tdx_buggy_shutdown(), wbinvd, etc? > >> > > >> > Not sure about this approach. When TDX module is buggy and the page is still > >> > accessible to guest as private pages, even with no-more SEAMCALLs flag, is it > >> > safe enough for guest_memfd/hugetlb to re-assign the page to allow simultaneous > >> > access in shared memory with potential private access from TD or TDX module? > >> > >> If no more seamcalls are allowed and all cpus are made to exit SEAM > >> mode then how can there be potential private access from TD or TDX > >> module? > > Not sure. As Kirill said "TDX module has creative ways to corrupt it" > > https://lore.kernel.org/all/zlxgzuoqwrbuf54wfqycnuxzxz2yduqtsjinr5uq4ss7iuk2rt@qaaolzwsy6ki/. > > > > Or, could TDX just set a page flag, like what for XEN > > > > /* XEN */ > > /* Pinned in Xen as a read-only pagetable page. */ > > PG_pinned = PG_owner_priv_1, > > > > e.g. > > PG_tdx_firmware_access = PG_owner_priv_1, > > > > Then, guest_memfd checks this flag on every zap and replace it with PG_hwpoison > > on behalf of TDX? > > I think this question probably arose because of a misunderstanding I > might have caused. I meant to set the HWpoison flag from the kernel, not > from within the TDX module. Please see [1]. I understood. But as Rick pointed out https://lore.kernel.org/all/04d3e455d07042a0ab8e244e6462d9011c914581.camel@intel.com/, Manually setting the poison flag in KVM's TDX code (in host kernel) seems risky. > In addition, if the TDX module (now referring specifically to the TDX > module and not the kernel) sets page flags, that won't work with Marking at per-folio level seems acceptable to me. > vmemmap-optimized folios. Setting a page flag on a vmemmap-optimized > folio will be setting the flag on a few pages. BTW, I have a concern regarding to the overhead vmemmap-optimization. In my system, with hugetlb_free_vmemmap=false, the TD boot time is around 30s; with hugetlb_free_vmemmap=true, the TD boot time is around 1m20s; > [1] https://lore.kernel.org/all/diqzplej4llh.fsf@ackerleytng-ctop.c.googlers.com/