Re: [RFC PATCH 0/5] Prototype for direct map awareness in page allocator

From: Mike Rapoport <rppt@kernel.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Rick P Edgecombe <rick.p.edgecombe@intel.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"peterz@infradead.org" <peterz@infradead.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"song@kernel.org" <song@kernel.org>,
	"dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
	"vbabka@suse.cz" <vbabka@suse.cz>,
	"x86@kernel.org" <x86@kernel.org>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH 0/5] Prototype for direct map awareness in page allocator
Date: Thu, 25 May 2023 23:37:40 +0300	[thread overview]
Message-ID: <20230525203740.GB4967@kernel.org> (raw)
In-Reply-To: <ZGe/Md3FXb1ihXwe@google.com>

On Fri, May 19, 2023 at 11:25:53AM -0700, Sean Christopherson wrote:
> On Fri, May 19, 2023, Mike Rapoport wrote:
> > On Fri, May 19, 2023 at 08:40:48AM -0700, Sean Christopherson wrote:
> > > On Thu, Mar 09, 2023, Mike Rapoport wrote:
> > > > On Thu, Mar 09, 2023 at 01:59:00AM +0000, Edgecombe, Rick P wrote:
> > > 
> > > Would it be possible to drop that assumption/requirement, i.e. allow allocation of
> > > __GFP_UNMAPPED without __GFP_ZERO?  At a glance, __GFP_UNMAPPED looks like it would
> > > be a great fit for backing guest memory, in particular for confidential VMs.  And
> > > for some flavors of CoCo, i.e. TDX, the trusted intermediary is responsible for
> > > zeroing/initializing guest memory as the untrusted host (kernel/KVM) doesn't have
> > > access to the guest's encryption key.  In other words, zeroing in the kernel would
> > > be unnecessary work.
> > 
> > Making and unmapped allocation without __GFP_ZERO shouldn't be a problem. 
> > 
> > However, using a gfp flag and hooking up into the free path in page
> > allocator have issues and preferably should be avoided.
> > 
> > Will something like unmapped_alloc() and unmapped_free() work for your
> > usecase?
> 
> Yep, I'm leaning more and more towards having KVM implement its own ioctl() for
> managing this type of memory.  Wiring that up to use dedicated APIs should be no
> problem.

Ok, I've dropped the GFP flag and made unmapped_pages_{alloc,free} the only
APIs.

Totally untested version of what I've got is here:
https://git.kernel.org/pub/scm/linux/kernel/git/rppt/linux.git/log/?h=unmapped-alloc/rfc-v2

I have some thoughts about adding support for 1G pages, but this is still
really vague at the moment.

> Thanks!

-- 
Sincerely yours,
Mike.