linux-kernel - Re: [PATCHv3 7/8] x86: Expose untagging mask in /proc/$PID/arch

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Date:   Mon, 27 Jun 2022 13:00:54 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Dave Hansen <dave.hansen@...el.com>, Will Deacon <will@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.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: [PATCHv3 7/8] x86: Expose untagging mask in
 /proc/$PID/arch_status

Hi Kirill,

Sorry, this fell through the cracks (thanks to Will for reminding me).

On Sat, Jun 11, 2022 at 04:28:30AM +0300, Kirill A. Shutemov wrote:
> On Fri, Jun 10, 2022 at 08:24:38AM -0700, Dave Hansen wrote:
> > On 6/10/22 07:35, Kirill A. Shutemov wrote:
> > > +/*
> > > + * Report architecture specific information
> > > + */
> > > +int proc_pid_arch_status(struct seq_file *m, struct pid_namespace *ns,
> > > +			struct pid *pid, struct task_struct *task)
> > > +{
> > > +	/*
> > > +	 * Report AVX512 state if the processor and build option supported.
> > > +	 */
> > > +	if (cpu_feature_enabled(X86_FEATURE_AVX512F))
> > > +		avx512_status(m, task);
> > > +
> > > +	seq_printf(m, "untag_mask:\t%#lx\n", mm_untag_mask(task->mm));
> > > +
> > > +	return 0;
> > > +}
> > 
> > Arch-specific gunk is great for, well, arch-specific stuff.  AVX-512 and
> > its, um, "quirks", really won't show up anywhere else.  But x86 isn't
> > even the first to be doing this address tagging business.
> > 
> > Shouldn't we be talking to the ARM folks about a common way to do this?
> 
> + Catalin, Will.
> 
> I guess we can expose the mask via proc for ARM too, but I'm not sure if
> we can unify interface further without breaking existing TBI users: TBI is
> enabled per-thread while LAM is per-process.

Hardware TBI is enabled for all user space at boot (it was like this
form the beginning). The TBI syscall interface is per-thread (TIF flag)
but it doesn't change any hardware behaviour. The mask is fixed in
hardware, unchangeable. I'm fine with reporting an untag_mask in a
common way, only that setting it won't be possible on arm64.

If arm64 ever gains support for a modifiable untag_mask, it's a good
chance it would be per mm as well since the controls for TBI are per
page table.

-- 
Catalin