linux-kernel - Re: [PATCH 0/5] x86: Impplement support for unaccepted memory

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Date:   Tue, 10 Aug 2021 10:36:21 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        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>,
        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>, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5] x86: Impplement support for unaccepted memory

On 8/10/21 10:31 AM, Kirill A. Shutemov wrote:
> On Tue, Aug 10, 2021 at 08:51:01AM -0700, Dave Hansen wrote:
>> Let's say we have a 128GB VM.  How much does faster does this approach
>> reach userspace than if all memory was accepted up front?  How much
>> memory _could_ have been accepted at the point userspace starts running?
> 
> Acceptance code is not optimized yet: we accept memory in 4k chunk which
> is very slow because hypercall overhead dominates the picture.
> 
> As of now, kernel boot time of 1 VCPU and 64TiB VM with upfront memory
> accept is >20 times slower than with this lazy memory accept approach.

That's a pretty big deal.

> The difference is going to be substantially lower once we get it optimized
> properly.

What does this mean?  Is this future work in the kernel or somewhere in
the TDX hardware/firmware which will speed things up?