linux-kernel - Re: [PATCH 1/6] KVM: guest_memfd: Add DEFAULT_SHARED flag, reject user page faults if not set

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Message-ID: <CAGtprH_JgWfr2wPGpJg_mY5Sxf6E0dp5r-_4aVLi96To2pugXA@mail.gmail.com>
Date: Wed, 1 Oct 2025 07:22:54 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Ackerley Tng <ackerleytng@...gle.com>, David Hildenbrand <david@...hat.com>, 
	Patrick Roy <patrick.roy@...ux.dev>, Fuad Tabba <tabba@...gle.com>, 
	Paolo Bonzini <pbonzini@...hat.com>, Christian Borntraeger <borntraeger@...ux.ibm.com>, 
	Janosch Frank <frankja@...ux.ibm.com>, Claudio Imbrenda <imbrenda@...ux.ibm.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Nikita Kalyazin <kalyazin@...zon.co.uk>, shivankg@....com
Subject: Re: [PATCH 1/6] KVM: guest_memfd: Add DEFAULT_SHARED flag, reject
 user page faults if not set

On Mon, Sep 29, 2025 at 5:15 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> Oh!  This got me looking at kvm_arch_supports_gmem_mmap() and thus
> KVM_CAP_GUEST_MEMFD_MMAP.  Two things:
>
>  1. We should change KVM_CAP_GUEST_MEMFD_MMAP into KVM_CAP_GUEST_MEMFD_FLAGS so
>     that we don't need to add a capability every time a new flag comes along,
>     and so that userspace can gather all flags in a single ioctl.  If gmem ever
>     supports more than 32 flags, we'll need KVM_CAP_GUEST_MEMFD_FLAGS2, but
>     that's a non-issue relatively speaking.
>

Guest_memfd capabilities don't necessarily translate into flags, so ideally:
1) There should be two caps, KVM_CAP_GUEST_MEMFD_FLAGS and
KVM_CAP_GUEST_MEMFD_CAPS.
2) IMO they should both support namespace of 64 values at least from the get go.
3) The reservation scheme for upstream should ideally be LSB's first
for the new caps/flags.

guest_memfd will achieve multiple features in future, both upstream
and in out-of-tree versions to deploy features before they make their
way upstream. Generally the scheme followed by out-of-tree versions is
to define a custom UAPI that won't conflict with upstream UAPIs in
near future. Having a namespace of 32 values gives little space to
avoid the conflict, e.g. features like hugetlb support will have to
eat up at least 5 bits from the flags [1].

[1] https://elixir.bootlin.com/linux/v6.17/source/include/uapi/asm-generic/hugetlb_encode.h#L20