linux-kernel - Re: [PATCH v3 3/5] arch/x86/mm: Refactor cond

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Message-ID: <87wo69us75.fsf@nanos.tec.linutronix.de>
Date:   Tue, 21 Apr 2020 11:02:54 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Singh\, Balbir" <sblbir@...zon.com>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     "keescook\@chromium.org" <keescook@...omium.org>,
        "tony.luck\@intel.com" <tony.luck@...el.com>,
        "benh\@kernel.crashing.org" <benh@...nel.crashing.org>,
        "jpoimboe\@redhat.com" <jpoimboe@...hat.com>,
        "x86\@kernel.org" <x86@...nel.org>,
        "dave.hansen\@intel.com" <dave.hansen@...el.com>
Subject: Re:  [PATCH v3 3/5] arch/x86/mm: Refactor cond_ibpb() to support other use cases

"Singh, Balbir" <sblbir@...zon.com> writes:
> On Sat, 2020-04-18 at 11:59 +0200, Thomas Gleixner wrote:
>> 
>> "Singh, Balbir" <sblbir@...zon.com> writes:
>> > On Fri, 2020-04-17 at 15:07 +0200, Thomas Gleixner wrote:
>> > > 
>> > > Balbir Singh <sblbir@...zon.com> writes:
>> > > > 
>> > > >  /*
>> > > > - * Use bit 0 to mangle the TIF_SPEC_IB state into the mm pointer
>> > > > which is
>> > > > - * stored in cpu_tlb_state.last_user_mm_ibpb.
>> > > > + * Bits to mangle the TIF_SPEC_IB state into the mm pointer which is
>> > > > + * stored in cpu_tlb_state.last_user_mm_spec.
>> > > >   */
>> > > >  #define LAST_USER_MM_IBPB    0x1UL
>> > > > +#define LAST_USER_MM_SPEC_MASK       (LAST_USER_MM_IBPB)
>> > > > 
>> > > >       /* Reinitialize tlbstate. */
>> > > > -     this_cpu_write(cpu_tlbstate.last_user_mm_ibpb,
>> > > > LAST_USER_MM_IBPB);
>> > > > +     this_cpu_write(cpu_tlbstate.last_user_mm_spec,
>> > > > LAST_USER_MM_IBPB);
>> > > 
>> > > Shouldn't that be LAST_USER_MM_MASK?
>> > > 
>> > 
>> > No, that crashes the system for SW flushes, because it tries to flush the
>> > L1D
>> > via the software loop and early enough we don't have the l1d_flush_pages
>> > allocated. LAST_USER_MM_MASK has LAST_USER_MM_FLUSH_L1D bit set.
>> 
>> You can trivially prevent this by checking l1d_flush_pages != NULL.
>> 
>
> But why would we want to flush on reinit? It is either coming back from a low
> power state or initialising, is it worth adding a check for != NULL everytime
> we flush to handle this case?

Fair enough. Please add a comment so the same question does not come
back 3 month from now.

Thanks,

        tglx