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

From: Michael Roth <michael.roth@amd.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: 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>,
	Vlastimil Babka <vbabka@suse.cz>,
	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: Wed, 2 Nov 2022 16:26:56 -0500	[thread overview]
Message-ID: <20221102212656.6giugw542jdxsvhh@amd.com> (raw)
In-Reply-To: <20221102211404.l5whyif3j3k67fv2@box.shutemov.name>

On Thu, Nov 03, 2022 at 12:14:04AM +0300, Kirill A. Shutemov wrote:
> On Mon, Oct 31, 2022 at 12:47:38PM -0500, Michael Roth wrote:
> > 
> > In v8 there was some discussion about potentially passing the page/folio
> > and order as part of the invalidation callback, I ended up needing
> > something similar for SEV-SNP, and think it might make sense for other
> > platforms. This main reasoning is:
> > 
> >   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)
> 
> That's news to me. Where the restriction for SNP comes from? There's no
> such limitation on TDX side AFAIK?
> 
> Could you point me to relevant documentation if there's any?

I could be mistaken, I haven't looked into the specific documentation and was
going off of this discussion from a ways back:

  https://lore.kernel.org/all/YWb8WG6Ravbs1nbx@google.com/

Sean, is my read of that correct? Do you happen to know where there's
some documentation on that for the TDX side?

Thanks,

Mike

> 
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov