linux-kernel - Re: [PATCHv7 08/14] x86/mm: Reserve unaccepted memory bitmap

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Date:   Thu, 1 Dec 2022 11:37:10 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Borislav Petkov <bp@...en8.de>,
        "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>,
        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 08/14] x86/mm: Reserve unaccepted memory bitmap

On Wed, Nov 30, 2022 at 04:28:40AM +0300, Kirill A. Shutemov wrote:
> On Tue, Jul 26, 2022 at 11:07:14AM +0200, Borislav Petkov wrote:
> > On Tue, Jun 14, 2022 at 03:02:25PM +0300, Kirill A. Shutemov wrote:
> > > diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
> > > index f267205f2d5a..22d1fe48dcba 100644
> > > --- a/arch/x86/kernel/e820.c
> > > +++ b/arch/x86/kernel/e820.c
> > > @@ -1316,6 +1316,16 @@ void __init e820__memblock_setup(void)
> > >  	int i;
> > >  	u64 end;
> > >  
> > > +	/* Mark unaccepted memory bitmap reserved */
> > > +	if (boot_params.unaccepted_memory) {
> > > +		unsigned long size;
> > > +
> > > +		/* One bit per 2MB */
> > > +		size = DIV_ROUND_UP(e820__end_of_ram_pfn() * PAGE_SIZE,
> > > +				    PMD_SIZE * BITS_PER_BYTE);
> > > +		memblock_reserve(boot_params.unaccepted_memory, size);
> > > +	}
> > > +
> > 
> > Hmm, I don't like how this is dropped right in the middle of a unrelated
> > function.
> > 
> > You're adding arch/x86/mm/unaccepted_memory.c later. Why don't you put
> > that chunk in a function there which is called by early_reserve_memory()
> > which does exactly what you want - reserve memory early, before memblock
> > allocations?
> 
> early_reserve_memory() specifically called before e820__memory_setup()
> (see comment in setup_arch()), so we don't have e820_table finalized and
> we need it to get correct RAM size from e820__end_of_ram_pfn().
> 
> I guess we can hide the chunk in a function in unaccepted_memory.c and
> call it from here, but it would require #ifdeffery in a header file as the
> .c is only compiled for CONFIG_UNACCEPTED_MEMORY=y.
> 
> Looks like an overkill to me, no?

Agree. Can we just extend the comment to explain why we reserve the bitmap
at e820__memblock_setup() rather than in early_reserve_memory(), pretty
much with the explanation above?
 
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov

-- 
Sincerely yours,
Mike.