linux-kernel - Re: I915 CI-run with kfence enabled, issues found

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Date:   Mon, 29 Mar 2021 14:03:40 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Marco Elver <elver@...gle.com>
Cc:     "Sarvela, Tomi P" <tomi.p.sarvela@...el.com>,
        "kasan-dev@...glegroups.com" <kasan-dev@...glegroups.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: I915 CI-run with kfence enabled, issues found

On 3/29/21 10:45 AM, Marco Elver wrote:
> On Mon, 29 Mar 2021 at 19:32, Dave Hansen <dave.hansen@...el.com> wrote:
> Doing it to all CPUs is too expensive, and we can tolerate this being
> approximate (nothing bad will happen, KFENCE might just miss a bug and
> that's ok).
...
>> BTW, the preempt checks in flush_tlb_one_kernel() are dependent on KPTI
>> being enabled.  That's probably why you don't see this everywhere.  We
>> should probably have unconditional preempt checks in there.
> 
> In which case I'll add a preempt_disable/enable() pair to
> kfence_protect_page() in arch/x86/include/asm/kfence.h.

That sounds sane to me.  I'd just plead that the special situation (not
needing deterministic TLB flushes) is obvious.  We don't want any folks
copying this code.

BTW, I know you want to avoid the cost of IPIs, but have you considered
any other low-cost ways to get quicker TLB flushes?  For instance, you
could loop over all CPUs and set cpu_tlbstate.invalidate_other=1.  That
would induce a context switch at the next context switch without needing
an IPI.