linux-kernel - Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

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Date:   Wed, 13 Apr 2022 13:32:21 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <jroedel@...e.de>,
        Ard Biesheuvel <ardb@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Varad Gautam <varad.gautam@...e.com>,
        Dario Faggioli <dfaggioli@...e.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Mike Rapoport <rppt@...nel.org>, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCHv4 1/8] mm: Add support for unaccepted memory

On 13.04.22 13:30, Kirill A. Shutemov wrote:
> On Wed, Apr 13, 2022 at 12:36:11PM +0200, David Hildenbrand wrote:
>> On 12.04.22 18:08, Dave Hansen wrote:
>>> On 4/12/22 01:15, David Hildenbrand wrote:
>>>> Can we simply automate this using a kthread or smth like that, which
>>>> just traverses the free page lists and accepts pages (similar, but
>>>> different to free page reporting)?
>>>
>>> That's definitely doable.
>>>
>>> The downside is that this will force premature consumption of physical
>>> memory resources that the guest may never use.  That's a particular
>>> problem on TDX systems since there is no way for a VMM to reclaim guest
>>> memory short of killing the guest.
>>
>> IIRC, the hypervisor will usually effectively populate all guest RAM
>> either way right now.
> 
> No, it is not usual. By default QEMU/KVM uses anonymous mapping and
> fault-in memory on demand.
> 
> Yes, there's an option to pre-populate guest memory, but it is not the
> default.

Let me be clearer: I'm talking about the TDX/SEV world, not ordinary
unencrypted VMs. For ordinary encrypted VMs we do have populate on
demand frequently.

For SEV we currently pin all guest memory and consequently don't have
populate on demand. For TDX, again, I did not follow how fd-based
private guest memory will behave. I thought I remembered that we will
similarly not have populate-on-demand.

Preallocation is usually used with huge pages, but I guess that's out of
scope right now for encrypted VMs.


-- 
Thanks,

David / dhildenb