linux-kernel - Re: [PATCHv5 08/12] x86/mm: Provide helpers for unaccepted memory

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Date:   Wed, 11 May 2022 11:07:08 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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>,
        Dave Hansen <dave.hansen@...el.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Mike Rapoport <rppt@...nel.org>,
        David Hildenbrand <david@...hat.com>, x86@...nel.org,
        linux-mm@...ck.org, linux-coco@...ts.linux.dev,
        linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv5 08/12] x86/mm: Provide helpers for unaccepted memory

On Wed, May 11, 2022 at 04:15:35AM +0300, Kirill A. Shutemov wrote:
> Okay. Fair enough. I will change it to
> 
> 			panic("Cannot accept memory: unknown platform.");

So I haven't went all the way in the patchset but what I see is:

                /* Platform-specific memory-acceptance call goes here */
                if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST)) {
                        tdx_accept_memory(range_start * PMD_SIZE,
                                          range_end * PMD_SIZE);
                } else {
                        panic("Cannot accept memory");
                }

so how would you decide for some other platform that it should panic?

TDX should panic, that I get. But you can just as well WARN_ONCE() here
so that it gets fixed. Panicking is counterproductive.

> It checks if the range of memory requires accept_memory() call before it
> can be accessed.
> 
> If any part of the range is not accepted, the call is required.
> accept_memory() knows what exactly has to be done. Note that
> accept_memory() call is harmless for any valid memory range.
> It can be called on already accepted memory.

Aaah, so that's what I was missing. So this function definitely needs a
comment ontop of it. And a name change to something like

	range_contains_unaccepted_memory()

or so to actually state what it does.

-- 
Regards/Gruss,
    Boris.

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