linux-kernel - Re: [PATCHv7 02/14] mm: Add support for unaccepted memory

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Date:   Fri, 22 Jul 2022 21:18:44 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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>,
        Mike Rapoport <rppt@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        marcelo.cerri@...onical.com, tim.gardner@...onical.com,
        khalid.elmously@...onical.com, philip.cox@...onical.com,
        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: [PATCHv7 02/14] mm: Add support for unaccepted memory

On Thu, Jul 21, 2022 at 08:49:31AM -0700, Dave Hansen wrote:
> Acceptance is slow and the heavy lifting is done inside the TDX module.
>  It involves flushing old aliases out of the caches and initializing the
> memory integrity metadata for every cacheline.  This implementation does
> acceptance in 2MB chunks while holding a global lock.

Oh, fun.

> So, those (effective) 2MB clflush+memset's (plus a few thousand cycles
> for the hypercall/tdcall transitions)

So this sounds strange - page validation on AMD - judging by the
pseudocode of the PVALIDATE insn - does a bunch of sanity checks on the
gVA of the page and then installs it into the RMP and also "PVALIDATE
performs the same segmentation and paging checks as a 1-byte read.
PVALIDATE does not invalidate TLB caches."

But that still sounds a lot less work than what the TDX module needs to
do...

> can't happen in parallel. They are serialized and must wait on each
> other.

Ofc, the Intel version of the RMP table needs to be protected. :-)

> If you have a few hundred CPUs all trying to allocate memory (say,
> doing the first kernel compile after a reboot), this is going to be
> very, very painful for a while.
>
> That said, I think this is the right place to _start_. There is going
> to need to be some kind of follow-on solution (likely background
> acceptance of some kind). But, even with that solution, *this* code
> is still needed to handle the degenerate case where the background
> accepter can't keep up with foreground memory needs.

I'm still catering to the view that it should be a two-tier thing: you
validate during boot a certain amount - say 4G - a size for which the
boot delay is acceptable and you do the rest on-demand along with a
background accepter.

That should give you the best of both worlds...

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette