linux-kernel - Re: [PATCH] x86/mm: Clean up the __[PHYSICAL/VIRTUAL]_MASK

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Date:   Thu, 9 May 2019 21:20:01 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Yury Norov <ynorov@...vell.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/mm: Clean up the __[PHYSICAL/VIRTUAL]_MASK_SHIFT
 definitions a bit


* Yury Norov <yury.norov@...il.com> wrote:

> On Thu, May 09, 2019 at 11:01:31AM +0200, Ingo Molnar wrote:
> > 
> > * Yury Norov <yury.norov@...il.com> wrote:
> > 
> > > __VIRTUAL_MASK_SHIFT is defined twice to the same valie in
> > > arch/x86/include/asm/page_32_types.h. Fix it.
> > > 
> > > Signed-off-by: Yury Norov <ynorov@...vell.com>
> > > ---
> > >  arch/x86/include/asm/page_32_types.h | 5 ++---
> > >  1 file changed, 2 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/arch/x86/include/asm/page_32_types.h b/arch/x86/include/asm/page_32_types.h
> > > index 0d5c739eebd7..9bfac5c80d89 100644
> > > --- a/arch/x86/include/asm/page_32_types.h
> > > +++ b/arch/x86/include/asm/page_32_types.h
> > > @@ -28,6 +28,8 @@
> > >  #define MCE_STACK 0
> > >  #define N_EXCEPTION_STACKS 1
> > >  
> > > +#define __VIRTUAL_MASK_SHIFT	32
> > > +
> > >  #ifdef CONFIG_X86_PAE
> > >  /*
> > >   * This is beyond the 44 bit limit imposed by the 32bit long pfns,
> > > @@ -36,11 +38,8 @@
> > >   * The real limit is still 44 bits.
> > >   */
> > >  #define __PHYSICAL_MASK_SHIFT	52
> > > -#define __VIRTUAL_MASK_SHIFT	32
> > > -
> > >  #else  /* !CONFIG_X86_PAE */
> > >  #define __PHYSICAL_MASK_SHIFT	32
> > > -#define __VIRTUAL_MASK_SHIFT	32
> > >  #endif	/* CONFIG_X86_PAE */
> > 
> > I think it's clearer to see them defined where the physical mask shift is 
> > defined.
> > 
> > How about the patch below? It does away with the weird formatting and 
> > cleans up both the comments and the style of the definition:
> > 
> > /*
> >  * 52 bits on PAE is beyond the 44-bit limit imposed by the
> >  * 32-bit long PFNs, but we need the full mask to make sure
> >  * inverted PROT_NONE entries have all the host bits set
> >  * in a guest. The real limit is still 44 bits.
> >  */
> > #ifdef CONFIG_X86_PAE
> > # define __PHYSICAL_MASK_SHIFT	52
> > # define __VIRTUAL_MASK_SHIFT	32
> > #else
> > # define __PHYSICAL_MASK_SHIFT	32
> > # define __VIRTUAL_MASK_SHIFT	32
> > #endif
> > 
> > ?
> 
> My main concern was about double definition. It pretty looks like a
> bug. But if it's intentional, I'm OK. In the patch below, could you
> please add some note to the comment that __VIRTUAL_MASK_SHIFT defined
> twice intentionally?

It's not defined "twice", it has values set in the PAE and the non-PAE 
branch - just like __PHYSICAL_MASK_SHIFT.

__VIRTUAL_MASK_SHIFT happens to have the same value in both branches, but 
that's OK and pretty standard and happens in other headers too.

Thanks,

	Ingo