Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Mon, 4 Jul 2022 16:43:46 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        "kcc@google.com" <kcc@google.com>,
        "ryabinin.a.a@gmail.com" <ryabinin.a.a@gmail.com>,
        "andreyknvl@gmail.com" <andreyknvl@gmail.com>,
        "glider@google.com" <glider@google.com>,
        "dvyukov@google.com" <dvyukov@google.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Andi Kleen <ak@linux.intel.com>,
        Rick P Edgecombe <rick.p.edgecombe@intel.com>,
        linux-mm@kvack.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and
 ARCH_ENABLE_TAGGED_ADDR
Message-ID: <20220704134346.ua37tgg2c5eff4jm@black.fi.intel.com>
References: <20220610143527.22974-1-kirill.shutemov@linux.intel.com>
 <20220610143527.22974-7-kirill.shutemov@linux.intel.com>
 <6cb17661-9436-afbf-38eb-58565bba1a56@kernel.org>
 <20220629005342.3thjt26e6p6znyrh@black.fi.intel.com>
 <1d765bc0-279c-4fd3-91f4-e99e6aef203c@www.fastmail.com>
 <20220701153840.7g55cazg73ukvr7l@black.fi.intel.com>
 <91ff4c04-ec26-418e-a685-f910505eec5a@www.fastmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <91ff4c04-ec26-418e-a685-f910505eec5a@www.fastmail.com>
Precedence: bulk

On Sat, Jul 02, 2022 at 04:55:40PM -0700, Andy Lutomirski wrote:
> > diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> > index 427ebef3f64b..cd2b03fe94c4 100644
> > --- a/arch/x86/kernel/process_64.c
> > +++ b/arch/x86/kernel/process_64.c
> > @@ -745,15 +745,16 @@ static long prctl_map_vdso(const struct 
> > vdso_image *image, unsigned long addr)
> >  static void enable_lam_func(void *mm)
> >  {
> >  	struct mm_struct *loaded_mm = this_cpu_read(cpu_tlbstate.loaded_mm);
> > +	unsigned long lam_mask;
> > 
> >  	if (loaded_mm != mm)
> >  		return;
> > 
> > -	/* Counterpart of smp_wmb() in prctl_enable_tagged_addr() */
> > -	smp_rmb();
> > +	lam_mask = READ_ONCE(loaded_mm->context.lam_cr3_mask);
> > 
> >  	/* Update CR3 to get LAM active on the CPU */
> > -	switch_mm(loaded_mm, loaded_mm, current);
> > +	write_cr3(__read_cr3() | lam_mask);
> 
> Perhaps this should also mask off the old LAM mask?

So far LAM enabling is one-way operation, so it should be fine.
But I think masking off is good idea to avoid problems in the future.

-- 
 Kirill A. Shutemov