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: <aN1h4XTfRsJ8dhVJ@google.com>
Date: Wed, 1 Oct 2025 10:16:17 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Vishal Annapurve <vannapurve@...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 Wed, Oct 01, 2025, Vishal Annapurve wrote:
> On Wed, Oct 1, 2025 at 9:15 AM Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > On Wed, Oct 01, 2025, Vishal Annapurve wrote:
> > > 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.
> >
> > I'm not saying we can't have another GUEST_MEMFD capability or three, all I'm
> > saying is that for enumerating what flags can be passed to KVM_CREATE_GUEST_MEMFD,
> > KVM_CAP_GUEST_MEMFD_FLAGS is a better fit than a one-off KVM_CAP_GUEST_MEMFD_MMAP.
> 
> Ah, ok. Then do you envision the guest_memfd caps to still be separate
> KVM caps per guest_memfd feature?

Yes?  No?  It depends on the feature and the actual implementation.  E.g.
KVM_CAP_IRQCHIP enumerates support for a whole pile of ioctls.