linux-kernel - Re: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory

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Message-ID: <ZUPIXt1XzZrriswG@google.com>
Date:   Thu, 2 Nov 2023 09:03:42 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Matlack <dmatlack@...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>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, 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-kernel@...r.kernel.org, Xiaoyao Li <xiaoyao.li@...el.com>,
        Xu Yilun <yilun.xu@...el.com>,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Anish Moorthy <amoorthy@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        "Mickaël Salaün" <mic@...ikod.net>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        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: [PATCH v13 16/35] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for
 guest-specific backing memory

On Thu, Nov 02, 2023, Paolo Bonzini wrote:
> On 10/31/23 23:39, David Matlack wrote:
> > > > Maybe can you sketch out how you see this proposal being extensible to
> > > > using guest_memfd for shared mappings?
> > > For in-place conversions, e.g. pKVM, no additional guest_memfd is needed.  What's
> > > missing there is the ability to (safely) mmap() guest_memfd, e.g. KVM needs to
> > > ensure there are no outstanding references when converting back to private.
> > > 
> > > For TDX/SNP, assuming we don't find a performant and robust way to do in-place
> > > conversions, a second fd+offset pair would be needed.
> > Is there a way to support non-in-place conversions within a single guest_memfd?
> 
> For TDX/SNP, you could have a hook from KVM_SET_MEMORY_ATTRIBUTES to guest
> memory.  The hook would invalidate now-private parts if they have a VMA,
> causing a SIGSEGV/EFAULT if the host touches them.
> 
> It would forbid mappings from multiple gfns to a single offset of the
> guest_memfd, because then the shared vs. private attribute would be tied to
> the offset.  This should not be a problem; for example, in the case of SNP,
> the RMP already requires a single mapping from host physical address to
> guest physical address.

I don't see how this can work.  It's not a M:1 scenario (where M is multiple gfns),
it's a 1:N scenario (wheren N is multiple offsets).  The *gfn* doesn't change on
a conversion, what needs to change to do non-in-place conversion is the pfn, which
is effectively the guest_memfd+offset pair.

So yes, we *could* support non-in-place conversions within a single guest_memfd,
but it would require a second offset, at which point it makes sense to add a
second file descriptor as well.  Userspace could still use a single guest_memfd
instance, i.e. pass in the same file descriptor but different offsets.