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Subject: Re: [PATCH 0/5] x86: Impplement support for unaccepted memory
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc:     "Kirill A. Shutemov" <kirill@shutemov.name>,
        Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Sean Christopherson <seanjc@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joerg Roedel <jroedel@suse.de>,
        Andi Kleen <ak@linux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Vlastimil Babka <vbabka@suse.cz>,
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        Thomas Gleixner <tglx@linutronix.de>,
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        Ingo Molnar <mingo@redhat.com>,
        Varad Gautam <varad.gautam@suse.com>,
        Dario Faggioli <dfaggioli@suse.com>, x86@kernel.org,
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        linux-kernel@vger.kernel.org
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <51d9168c-ac14-0907-79b3-5d4dd46f92d6@intel.com>
Date:   Tue, 10 Aug 2021 10:36:21 -0700
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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?