Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same
 user process
To:     Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>
Cc:     Arjan van de Ven <arjan@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        KarimAllah Ahmed <karahmed@amazon.de>,
        Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Asit Mallick <asit.k.mallick@intel.com>,
        Borislav Petkov <bp@suse.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "H . Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@amd.com>,
        Joerg Roedel <joro@8bytes.org>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Laura Abbott <labbott@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim Krcmar <rkrcmar@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tom Lendacky <thomas.lendacky@amd.com>, X86 ML <x86@kernel.org>
References: <c5cc429013c2248ddebe0a8480b172ae70b29733.1516840211.git.tim.c.chen@linux.intel.com>
 <20180125085820.GV2228@hirez.programming.kicks-ass.net>
 <20180125092233.GE2295@hirez.programming.kicks-ass.net>
 <86541aca-8de7-163d-b620-083dddf29184@linux.intel.com>
 <20180125135055.GK2249@hirez.programming.kicks-ass.net>
 <aeef4a08-7dd3-3d3a-02f4-9b8b4a1461bf@linux.intel.com>
 <20180125164139.GM2269@hirez.programming.kicks-ass.net>
 <CALCETrVeb7g+wAP8hox9TEBBWYAFXGzOv5t2aQVrtuW4D6NQQQ@mail.gmail.com>
 <20180125181852.GL2249@hirez.programming.kicks-ass.net>
From:   Tim Chen <tim.c.chen@linux.intel.com>
Message-ID: <aa7cfe53-d8ee-9f56-a5c7-775d878cee15@linux.intel.com>
Date:   Thu, 25 Jan 2018 11:32:46 -0800
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.1.0
MIME-Version: 1.0
In-Reply-To: <20180125181852.GL2249@hirez.programming.kicks-ass.net>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 01/25/2018 10:18 AM, Peter Zijlstra wrote:
> On Thu, Jan 25, 2018 at 09:04:21AM -0800, Andy Lutomirski wrote:
>> I haven't tried to fully decipher the patch, but I think the idea is
>> wrong.  (I think it's the same wrong idea that Rik and I both had and
>> that I got into Linus' tree for a while...)  The problem is that it's
>> not actually correct to run indefinitely in kernel mode using stale
>> cached page table data.  The stale PTEs themselves are fine, but the
>> stale intermediate translations can cause the CPU to speculatively
>> load complete garbage into the TLB, and that's bad (and causes MCEs on
>> AMD CPUs).
> 
> Urggh.. indeed :/
> 
>> I think we only really have two choices: tlb_defer_switch_to_init_mm()
>> == true and tlb_defer_switch_to_init_mm() == false.  The current
>> heuristic is to not defer if we have PCID, because loading CR3 is
>> reasonably fast.
> 
> I just _really_ _really_ hate idle drivers doing leave_mm(). I don't
> suppose limiting the !IPI case to just the idle case would be correct
> either, because between waking from idle and testing our 'should I have
> invalidated' bit it can (however unlikely) speculate into stale TLB
> entries too..
> 
> 

Peter, 

This patch is not ideal as it comes with the caveats that
patch 2 tries to close.  I put it out here to see if it can prompt
people to come up with a better solution. Keeping active_mm around would
have been cleaner but it looks like there are issues that Andy mentioned.

The "A -> idle -> A" case would not trigger IBPB if tlb_defer_switch_to_init_mm()
is true (non pcid) as we does not change the mm.

This patch tries to address the case when we do switch to init_mm and back.
Do you still have objections to the approach in this patch
to save the last active mm before switching to init_mm?

Tim