Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to create restricted user memory

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Michael Roth <michael.roth@amd.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-arch@vger.kernel.org, linux-api@vger.kernel.org,
	linux-doc@vger.kernel.org, qemu-devel@nongnu.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Sean Christopherson <seanjc@google.com>,
	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>,
	Hugh Dickins <hughd@google.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>,
	Vishal Annapurve <vannapurve@google.com>,
	Yu Zhang <yu.c.zhang@linux.intel.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	luto@kernel.org, jun.nakajima@intel.com, dave.hansen@intel.com,
	ak@linux.intel.com, david@redhat.com, aarcange@redhat.com,
	ddutile@redhat.com, dhildenb@redhat.com,
	Quentin Perret <qperret@google.com>,
	tabba@google.com, mhocko@suse.com,
	Muchun Song <songmuchun@bytedance.com>,
	wei.w.wang@intel.com
Subject: Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to create restricted user memory
Date: Mon, 14 Nov 2022 18:28:43 +0300	[thread overview]
Message-ID: <20221114152843.ylxe4dis254vrj5u@box.shutemov.name> (raw)
In-Reply-To: <20a11042-2cfb-8f42-9d80-6672e155ca2c@suse.cz>

On Mon, Nov 14, 2022 at 03:02:37PM +0100, Vlastimil Babka wrote:
> On 11/1/22 16:19, Michael Roth wrote:
> > On Tue, Nov 01, 2022 at 07:37:29PM +0800, Chao Peng wrote:
> >> > 
> >> >   1) restoring kernel directmap:
> >> > 
> >> >      Currently SNP (and I believe TDX) need to either split or remove kernel
> >> >      direct mappings for restricted PFNs, since there is no guarantee that
> >> >      other PFNs within a 2MB range won't be used for non-restricted
> >> >      (which will cause an RMP #PF in the case of SNP since the 2MB
> >> >      mapping overlaps with guest-owned pages)
> >> 
> >> Has the splitting and restoring been a well-discussed direction? I'm
> >> just curious whether there is other options to solve this issue.
> > 
> > For SNP it's been discussed for quite some time, and either splitting or
> > removing private entries from directmap are the well-discussed way I'm
> > aware of to avoid RMP violations due to some other kernel process using
> > a 2MB mapping to access shared memory if there are private pages that
> > happen to be within that range.
> > 
> > In both cases the issue of how to restore directmap as 2M becomes a
> > problem.
> > 
> > I was also under the impression TDX had similar requirements. If so,
> > do you know what the plan is for handling this for TDX?
> > 
> > There are also 2 potential alternatives I'm aware of, but these haven't
> > been discussed in much detail AFAIK:
> > 
> > a) Ensure confidential guests are backed by 2MB pages. shmem has a way to
> >    request 2MB THP pages, but I'm not sure how reliably we can guarantee
> >    that enough THPs are available, so if we went that route we'd probably
> >    be better off requiring the use of hugetlbfs as the backing store. But
> >    obviously that's a bit limiting and it would be nice to have the option
> >    of using normal pages as well. One nice thing with invalidation
> >    scheme proposed here is that this would "Just Work" if implement
> >    hugetlbfs support, so an admin that doesn't want any directmap
> >    splitting has this option available, otherwise it's done as a
> >    best-effort.
> > 
> > b) Implement general support for restoring directmap as 2M even when
> >    subpages might be in use by other kernel threads. This would be the
> >    most flexible approach since it requires no special handling during
> >    invalidations, but I think it's only possible if all the CPA
> >    attributes for the 2M range are the same at the time the mapping is
> >    restored/unsplit, so some potential locking issues there and still
> >    chance for splitting directmap over time.
> 
> I've been hoping that
> 
> c) using a mechanism such as [1] [2] where the goal is to group together
> these small allocations that need to increase directmap granularity so
> maximum number of large mappings are preserved.

As I mentioned in the other thread the restricted memfd can be backed by
secretmem instead of plain memfd. It already handles directmap with care.

But I don't think it has to be part of initial restricted memfd
implementation. It is SEV-specific requirement and AMD folks can extend
implementation as needed later.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov