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

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Message-ID: <20230525203740.GB4967@kernel.org>
Date:   Thu, 25 May 2023 23:37:40 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Rick P Edgecombe <rick.p.edgecombe@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "song@...nel.org" <song@...nel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        "x86@...nel.org" <x86@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 0/5] Prototype for direct map awareness in page
 allocator

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.