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AJvYcCUFpxEHSaImeZhVmUe0Oi0KnkCFccoxLYnOaMhbzZ5+vSORnj7et6u/ckZTBQYX2D4BQRp9I0FCnnSoSXJA@vger.kernel.org X-Gm-Message-State: AOJu0YwqrV3KF3iaDFF97vO+zFB22K1WtSIZ/KGR42sXEY+WIGHhcpF9 ydMkTBQ7um4qSWwem6FgctJqJtIkrph/XflfK+015Z/tkWz3Oa/j728hMWlOcqq6+7nipd6Sfdl ipDyOgAxTNlXlQURi1DPNQrNb60TvWo7Z6fbeRzPQ X-Gm-Gg: ASbGncvwkdz3srQ9s6Jjx3z7LE1cqRSWOYz29tZxTBsDdQH3fPrZ/fu53WYo9EvCiN3 /lqaCA6BWEbG4SOwff/Rixgl05iFHuIq1sp/6F68VGxm2k2i/n5aCP5KeULtSzzGN2kUsiOU0yr KLKZt3a7f3SvrMAibVvtPuZ3utbg7vCmYnk0KypoQXCxpWzYuqIsQceMcVfoQm4tHxV2ZnlQuSO k9TTO2WIotJ X-Google-Smtp-Source: AGHT+IFppI8EWFIKjUXPtViP+KrA09dQbTHMiXzo/hVpg329+fXbE9z6q1NN0DYucfeQOt83ReE7/FOzhTi0IjwwLl4= X-Received: by 2002:a17:903:320d:b0:216:48d4:b3a8 with SMTP id d9443c01a7336-23dda3cf61dmr1101735ad.16.1752015303037; Tue, 08 Jul 2025 15:55:03 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <006899ccedf93f45082390460620753090c01914.camel@intel.com> In-Reply-To: From: Vishal Annapurve Date: Tue, 8 Jul 2025 15:54:49 -0700 X-Gm-Features: Ac12FXyERUa_xOAJgG7dIrmMf4kvAAP-pOIx0wDj5UJ0uC1eIt2Bz0JJ6_rlubA Message-ID: Subject: Re: [RFC PATCH v2 00/51] 1G page support for guest_memfd To: Sean Christopherson Cc: Rick P Edgecombe , "pvorel@suse.cz" , "kvm@vger.kernel.org" , "catalin.marinas@arm.com" , Jun Miao , "nsaenz@amazon.es" , Kirill Shutemov , "pdurrant@amazon.co.uk" , "peterx@redhat.com" , "x86@kernel.org" , "tabba@google.com" , "amoorthy@google.com" , "quic_svaddagi@quicinc.com" , "jack@suse.cz" , "vkuznets@redhat.com" , "quic_eberman@quicinc.com" , "keirf@google.com" , "mail@maciej.szmigiero.name" , "anthony.yznaga@oracle.com" , Wei W Wang , "palmer@dabbelt.com" , "Wieczor-Retman, Maciej" , Yan Y Zhao , "ajones@ventanamicro.com" , "willy@infradead.org" , "paul.walmsley@sifive.com" , Dave Hansen , "aik@amd.com" , "usama.arif@bytedance.com" , "quic_mnalajal@quicinc.com" , "fvdl@google.com" , "rppt@kernel.org" , "quic_cvanscha@quicinc.com" , "maz@kernel.org" , "vbabka@suse.cz" , "anup@brainfault.org" , "thomas.lendacky@amd.com" , "linux-kernel@vger.kernel.org" , "mic@digikod.net" , "oliver.upton@linux.dev" , Fan Du , "akpm@linux-foundation.org" , "steven.price@arm.com" , "muchun.song@linux.dev" , "binbin.wu@linux.intel.com" , Zhiquan1 Li , "rientjes@google.com" , "mpe@ellerman.id.au" , Erdem Aktas , "david@redhat.com" , "jgg@ziepe.ca" , "hughd@google.com" , "jhubbard@nvidia.com" , Haibo1 Xu , Isaku Yamahata , "jthoughton@google.com" , "steven.sistare@oracle.com" , "quic_pheragu@quicinc.com" , "jarkko@kernel.org" , "chenhuacai@kernel.org" , Kai Huang , "shuah@kernel.org" , "bfoster@redhat.com" , "dwmw@amazon.co.uk" , Chao P Peng , "pankaj.gupta@amd.com" , Alexander Graf , "nikunj@amd.com" , "viro@zeniv.linux.org.uk" , "pbonzini@redhat.com" , "yuzenghui@huawei.com" , "jroedel@suse.de" , "suzuki.poulose@arm.com" , "jgowans@amazon.com" , Yilun Xu , "liam.merwick@oracle.com" , "michael.roth@amd.com" , "quic_tsoni@quicinc.com" , Xiaoyao Li , "aou@eecs.berkeley.edu" , Ira Weiny , "richard.weiyang@gmail.com" , "kent.overstreet@linux.dev" , "qperret@google.com" , "dmatlack@google.com" , "james.morse@arm.com" , "brauner@kernel.org" , "roypat@amazon.co.uk" , "linux-fsdevel@vger.kernel.org" , "ackerleytng@google.com" , "pgonda@google.com" , "quic_pderrin@quicinc.com" , "hch@infradead.org" , "will@kernel.org" , "linux-mm@kvack.org" Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Tue, Jul 8, 2025 at 12:59=E2=80=AFPM Sean Christopherson wrote: > > On Tue, Jul 08, 2025, Vishal Annapurve wrote: > > On Tue, Jul 8, 2025 at 11:03=E2=80=AFAM Sean Christopherson wrote: > > Few points that seem important here: > > 1) Userspace can and should be able to only dictate if memory contents > > need to be preserved on shared to private conversion. > > No, I was wrong, pKVM has use cases where it's desirable to preserve data= on > private =3D> shared conversions. > > Side topic, if you're going to use fancy indentation, align the indentati= on so > it's actually readable. > > > -> For SNP/TDX VMs: > > * Only usecase for preserving contents is initial memory > > population, which can be achieved by: > > - Userspace converting the ranges to shared, populating = the contents, > > converting them back to private and then calling SNP/T= DX specific > > existing ABI functions. > > * For runtime conversions, guest_memfd can't ensure memory conte= nts are > > preserved during shared to private conversions as the architec= tures > > don't support that behavior. > > * So IMO, this "preserve" flag doesn't make sense for SNP/TDX VM= s, even > > It makes sense, it's just not supported by the architecture *at runtime*.= Case > in point, *something* needs to allow preserving data prior to launching t= he VM. > If we want to go with the PRIVATE =3D> SHARED =3D> FILL =3D> PRIVATE appr= oach for TDX > and SNP, then we'll probably want to allow PRESERVE only until the VM ima= ge is > finalized. Maybe we can simplify the story a bit here for today, how about: 1) For shared to private conversions: * Is it safe to say that the conversion itself is always content preserving, it's upto the architecture what it does with memory contents on the private fa= ults? - During initial memory setup, userspace can control how private memory would be faulted in by architecture supported ABI operations. - After initial memory setup, userspace can't control how private memory would be faulted in. 2) For private to shared conversions: * Architecture decides what should be done with the memory on shared faults. - guest_memfd can query architecture whether to zero memory or not. -> guest_memfd will only take on the responsibility of zeroing if needed by the architecture on shared faults. -> Architecture is responsible for the behavior on private faults. In future, if there is a usecase for controlling runtime behavior of private faults, architecture can expose additional ABI that userspace can use after initiating guest_memfd conversion.