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Date: Thu, 2 Jun 2022 18:07:33 +0800 From: Chao Peng <chao.p.peng@...ux.intel.com> To: "Gupta, Pankaj" <pankaj.gupta@....com> Cc: Vishal Annapurve <vannapurve@...gle.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org, linux-doc@...r.kernel.org, qemu-devel@...gnu.org, Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, Sean Christopherson <seanjc@...gle.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, Wanpeng Li <wanpengli@...cent.com>, Jim Mattson <jmattson@...gle.com>, Joerg Roedel <joro@...tes.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>, Hugh Dickins <hughd@...gle.com>, Jeff Layton <jlayton@...nel.org>, "J . Bruce Fields" <bfields@...ldses.org>, Andrew Morton <akpm@...ux-foundation.org>, Mike Rapoport <rppt@...nel.org>, Steven Price <steven.price@....com>, "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>, Vlastimil Babka <vbabka@...e.cz>, Yu Zhang <yu.c.zhang@...ux.intel.com>, "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, Jun Nakajima <jun.nakajima@...el.com>, dave.hansen@...el.com, ak@...ux.intel.com, david@...hat.com, aarcange@...hat.com, ddutile@...hat.com, dhildenb@...hat.com, Quentin Perret <qperret@...gle.com>, Michael Roth <michael.roth@....com>, mhocko@...e.com Subject: Re: [PATCH v6 3/8] mm/memfd: Introduce MFD_INACCESSIBLE flag On Wed, Jun 01, 2022 at 02:11:42PM +0200, Gupta, Pankaj wrote: > > > > > Introduce a new memfd_create() flag indicating the content of the > > > > created memfd is inaccessible from userspace through ordinary MMU > > > > access (e.g., read/write/mmap). However, the file content can be > > > > accessed via a different mechanism (e.g. KVM MMU) indirectly. > > > > > > > > > > SEV, TDX, pkvm and software-only VMs seem to have usecases to set up > > > initial guest boot memory with the needed blobs. > > > TDX already supports a KVM IOCTL to transfer contents to private > > > memory using the TDX module but rest of the implementations will need > > > to invent > > > a way to do this. > > > > There are some discussions in https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flkml.org%2Flkml%2F2022%2F5%2F9%2F1292&data=05%7C01%7Cpankaj.gupta%40amd.com%7Cb81ef334e2dd44c6143308da43b87d17%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637896756895977587%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=oQbM2Hj7GlhJTwnTM%2FPnwsfJlmTL7JR9ULBysAqm6V8%3D&reserved=0 > > already. I somehow agree with Sean. TDX is using an dedicated ioctl to > > copy guest boot memory to private fd so the rest can do that similarly. > > The concern is the performance (extra memcpy) but it's trivial since the > > initial guest payload is usually optimized in size. > > > > > > > > Is there a plan to support a common implementation for either allowing > > > initial write access from userspace to private fd or adding a KVM > > > IOCTL to transfer contents to such a file, > > > as part of this series through future revisions? > > > > Indeed, adding pre-boot private memory populating on current design > > isn't impossible, but there are still some opens, e.g. how to expose > > private fd to userspace for access, pKVM and CC usages may have > > different requirements. Before that's well-studied I would tend to not > > add that and instead use an ioctl to copy. Whether we need a generic > > ioctl or feature-specific ioctl, I don't have strong opinion here. > > Current TDX uses a feature-specific ioctl so it's not covered in this > > series. > > Common function or ioctl to populate preboot private memory actually makes > sense. > > Sorry, did not follow much of TDX code yet, Is it possible to filter out > the current TDX specific ioctl to common function so that it can be used by > other technologies? TDX code is here: https://patchwork.kernel.org/project/kvm/patch/70ed041fd47c1f7571aa259450b3f9244edda48d.1651774250.git.isaku.yamahata@intel.com/ AFAICS It might be possible to filter that out to a common function. But would like to hear from Paolo/Sean for their opinion. Chao > > Thanks, > Pankaj
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