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

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Date:   Fri, 22 Jul 2022 12:30:36 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Borislav Petkov <bp@...en8.de>
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 7/22/22 12:18, Borislav Petkov wrote:
> On Thu, Jul 21, 2022 at 08:49:31AM -0700, Dave Hansen wrote:
>> 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...

Sure does...  *Something* has to manage the cache coherency so that old
physical aliases of the converted memory don't write back and clobber
new data.  But, maybe the hardware is doing that now.


>> 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...

Yeah, that two-tier system is the way it's happening today from what I
understand.  This whole conversation is about how to handle the >4GB memory.