linux-kernel - Re: [RFCv2 08/10] x86/mm: Make LAM_U48 and mappings above 47-bits mutually exclusive

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Date:   Wed, 18 May 2022 20:08:15 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Bharata B Rao <bharata@....com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFCv2 08/10] x86/mm: Make LAM_U48 and mappings above 47-bits
 mutually exclusive

On Wed, May 18, 2022 at 02:13:06PM +0530, Bharata B Rao wrote:
> On 5/11/2022 7:57 AM, Kirill A. Shutemov wrote:
> > LAM_U48 steals bits above 47-bit for tags and makes it impossible for
> > userspace to use full address space on 5-level paging machine.
> > 
> > Make these features mutually exclusive: whichever gets enabled first
> > blocks the othe one.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  arch/x86/include/asm/elf.h         |  3 ++-
> >  arch/x86/include/asm/mmu_context.h | 13 +++++++++++++
> >  arch/x86/kernel/sys_x86_64.c       |  5 +++--
> >  arch/x86/mm/hugetlbpage.c          |  6 ++++--
> >  arch/x86/mm/mmap.c                 |  9 ++++++++-
> >  5 files changed, 30 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
> > index 29fea180a665..53b96b0c8cc3 100644
> > --- a/arch/x86/include/asm/elf.h
> > +++ b/arch/x86/include/asm/elf.h
> > @@ -328,7 +328,8 @@ static inline int mmap_is_ia32(void)
> >  extern unsigned long task_size_32bit(void);
> >  extern unsigned long task_size_64bit(int full_addr_space);
> >  extern unsigned long get_mmap_base(int is_legacy);
> > -extern bool mmap_address_hint_valid(unsigned long addr, unsigned long len);
> > +extern bool mmap_address_hint_valid(struct mm_struct *mm,
> > +				    unsigned long addr, unsigned long len);
> >  extern unsigned long get_sigframe_size(void);
> >  
> >  #ifdef CONFIG_X86_32
> > diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
> > index 27516046117a..c8a6d80dfec3 100644
> > --- a/arch/x86/include/asm/mmu_context.h
> > +++ b/arch/x86/include/asm/mmu_context.h
> > @@ -218,6 +218,19 @@ static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
> >  
> >  unsigned long __get_current_cr3_fast(void);
> >  
> > +#ifdef CONFIG_X86_5LEVEL
> > +static inline bool full_va_allowed(struct mm_struct *mm)
> > +{
> > +	/* LAM_U48 steals VA bits abouve 47-bit for tags */
> > +	return mm->context.lam != LAM_U48;
> > +}
> > +#else
> 
> This is called from X86 common code but appears to be LAM-specific.
> What would mm->context.lam contain if X86_FEATURE_LAM isn't set?

0. So full_va_allowed() will always return true.

-- 
 Kirill A. Shutemov