linux-kernel - Re: [PATCH v12 04/10] set_memory: allow querying whether set_direct_map

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Message-ID: <20201125121138.GJ123287@linux.ibm.com>
Date:   Wed, 25 Nov 2020 14:11:38 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Mike Rapoport <rppt@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Roman Gushchin <guro@...com>, Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, Will Deacon <will@...nel.org>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-riscv@...ts.infradead.org,
        x86@...nel.org
Subject: Re: [PATCH v12 04/10] set_memory: allow querying whether
 set_direct_map_*() is actually enabled

On Wed, Nov 25, 2020 at 12:22:52PM +0100, David Hildenbrand wrote:
> >  #include <asm-generic/cacheflush.h>
> >  
> >  #endif /* __ASM_CACHEFLUSH_H */
> > diff --git a/arch/arm64/include/asm/set_memory.h b/arch/arm64/include/asm/set_memory.h
> > new file mode 100644
> > index 000000000000..ecb6b0f449ab
> > --- /dev/null
> > +++ b/arch/arm64/include/asm/set_memory.h
> > @@ -0,0 +1,17 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +
> > +#ifndef _ASM_ARM64_SET_MEMORY_H
> > +#define _ASM_ARM64_SET_MEMORY_H
> > +
> > +#include <asm-generic/set_memory.h>
> > +
> > +bool can_set_direct_map(void);
> > +#define can_set_direct_map can_set_direct_map
> 
> Well, that looks weird.
> [...]

We have a lot of those e.g. in linux/pgtable.h

> >  }
> > +#else /* CONFIG_ARCH_HAS_SET_DIRECT_MAP */
> > +/*
> > + * Some architectures, e.g. ARM64 can disable direct map modifications at
> > + * boot time. Let them overrive this query.
> > + */
> > +#ifndef can_set_direct_map
> > +static inline bool can_set_direct_map(void)
> > +{
> > +	return true;
> > +}
> 
> I think we prefer __weak functions for something like that, avoids the
> ifdefery.

I'd prefer this for two reasons: first, static inline can be optimized
away and second, there is no really good place to put generic
implementation.

> Apart from that, LGTM.
> 
> -- 
> Thanks,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.