Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory

From: Sean Christopherson <seanjc@google.com>
To: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Chao Peng <chao.p.peng@linux.intel.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-api@vger.kernel.org, linux-doc@vger.kernel.org,
	qemu-devel@nongnu.org, linux-kselftest@vger.kernel.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
	Jeff Layton <jlayton@kernel.org>,
	"J . Bruce Fields" <bfields@fieldses.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Shuah Khan <shuah@kernel.org>, Mike Rapoport <rppt@kernel.org>,
	Steven Price <steven.price@arm.com>,
	"Maciej S . Szmigiero" <mail@maciej.szmigiero.name>,
	Vlastimil Babka <vbabka@suse.cz>,
	Vishal Annapurve <vannapurve@google.com>,
	Yu Zhang <yu.c.zhang@linux.intel.com>,
	luto@kernel.org, jun.nakajima@intel.com, dave.hansen@intel.com,
	ak@linux.intel.com, aarcange@redhat.com, ddutile@redhat.com,
	dhildenb@redhat.com, Quentin Perret <qperret@google.com>,
	Michael Roth <michael.roth@amd.com>,
	mhocko@suse.com, Muchun Song <songmuchun@bytedance.com>,
	"Gupta, Pankaj" <pankaj.gupta@amd.com>
Subject: Re: [PATCH v7 00/14] KVM: mm: fd-based approach for supporting KVM guest private memory
Date: Tue, 23 Aug 2022 16:05:27 +0000	[thread overview]
Message-ID: <YwT6x2g9jcMH60LI@google.com> (raw)
In-Reply-To: <b2743a3a-a1b4-2d2e-98be-87b58ad387cf@redhat.com>

On Tue, Aug 23, 2022, David Hildenbrand wrote:
> On 19.08.22 05:38, Hugh Dickins wrote:
> > On Fri, 19 Aug 2022, Sean Christopherson wrote:
> >> On Thu, Aug 18, 2022, Kirill A . Shutemov wrote:
> >>> On Wed, Aug 17, 2022 at 10:40:12PM -0700, Hugh Dickins wrote:
> >>>> On Wed, 6 Jul 2022, Chao Peng wrote:
> >>>> But since then, TDX in particular has forced an effort into preventing
> >>>> (by flags, seals, notifiers) almost everything that makes it shmem/tmpfs.
> >>>>
> >>>> Are any of the shmem.c mods useful to existing users of shmem.c? No.
> >>>> Is MFD_INACCESSIBLE useful or comprehensible to memfd_create() users? No.
> >>
> >> But QEMU and other VMMs are users of shmem and memfd.  The new features certainly
> >> aren't useful for _all_ existing users, but I don't think it's fair to say that
> >> they're not useful for _any_ existing users.
> > 
> > Okay, I stand corrected: there exist some users of memfd_create()
> > who will also have use for "INACCESSIBLE" memory.
> 
> As raised in reply to the relevant patch, I'm not sure if we really have
> to/want to expose MFD_INACCESSIBLE to user space. I feel like this is a
> requirement of specific memfd_notifer (memfile_notifier) implementations
> -- such as TDX that will convert the memory and MCE-kill the machine on
> ordinary write access. We might be able to set/enforce this when
> registering a notifier internally instead, and fail notifier
> registration if a condition isn't met (e.g., existing mmap).
>
> So I'd be curious, which other users of shmem/memfd would benefit from
> (MMU)-"INACCESSIBLE" memory obtained via memfd_create()?

I agree that there's no need to expose the inaccessible behavior via uAPI.  Making
it a kernel-internal thing that's negotiated/resolved when KVM binds to the fd
would align INACCESSIBLE with the UNMOVABLE and UNRECLAIMABLE flags (and any other
flags that get added in the future).

AFAICT, the user-visible flag is a holdover from the early RFCs and doesn't provide
any unique functionality.

If we go that route, we might want to have shmem/memfd require INACCESSIBLE to be
set for the initial implementation.  I.e. disallow binding without INACCESSIBLE
until there's a use case.