linux-kernel - Re: [RFC PATCH v11 08/29] KVM: Introduce per-page memory attributes

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Message-ID: <0c033063-5d20-4522-87e2-80ad3cca3602@redhat.com>
Date:   Fri, 21 Jul 2023 12:57:07 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>
Cc:     kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        kvmarm@...ts.linux.dev, linux-mips@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org,
        linux-riscv@...ts.infradead.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Wang <wei.w.wang@...el.com>,
        Liam Merwick <liam.merwick@...cle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH v11 08/29] KVM: Introduce per-page memory attributes

On 7/19/23 01:44, Sean Christopherson wrote:
> From: Chao Peng <chao.p.peng@...ux.intel.com>
> 
> In confidential computing usages, whether a page is private or shared is
> necessary information for KVM to perform operations like page fault
> handling, page zapping etc. There are other potential use cases for
> per-page memory attributes, e.g. to make memory read-only (or no-exec,
> or exec-only, etc.) without having to modify memslots.
> 
> Introduce two ioctls (advertised by KVM_CAP_MEMORY_ATTRIBUTES) to allow
> userspace to operate on the per-page memory attributes.
>    - KVM_SET_MEMORY_ATTRIBUTES to set the per-page memory attributes to
>      a guest memory range.
>    - KVM_GET_SUPPORTED_MEMORY_ATTRIBUTES to return the KVM supported
>      memory attributes.
> 
> Use an xarray to store the per-page attributes internally, with a naive,
> not fully optimized implementation, i.e. prioritize correctness over
> performance for the initial implementation.
> 
> Because setting memory attributes is roughly analogous to mprotect() on
> memory that is mapped into the guest, zap existing mappings prior to
> updating the memory attributes.  Opportunistically provide an arch hook
> for the post-set path (needed to complete invalidation anyways) in
> anticipation of x86 needing the hook to update metadata related to
> determining whether or not a given gfn can be backed with various sizes
> of hugepages.
> 
> It's possible that future usages may not require an invalidation, e.g.
> if KVM ends up supporting RWX protections and userspace grants _more_
> protections, but again opt for simplicity and punt optimizations to
> if/when they are needed.
> 
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Link: https://lore.kernel.org/all/Y2WB48kD0J4VGynX@google.com
> Cc: Fuad Tabba <tabba@...gle.com>
> Signed-off-by: Chao Peng <chao.p.peng@...ux.intel.com>
> Co-developed-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>