From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <peterz@infradead.org>
X-Google-Smtp-Source: AH8x224A38+tODB5PMbi2Fiy7aR4qko85ME0HTVikEI02pMEkQ3SKI0BSRK88mCSgiFXRfcAaAOm
ARC-Seal: i=1; a=rsa-sha256; t=1516904348; cv=none;
        d=google.com; s=arc-20160816;
        b=t8dVBA58Rm7KpKol1LQt97H+JMN99BTfhC+FkjzDtIPa0to0MSmlsrc+1Rr1AahsvF
         D6fAi82Xb3lPClQUcNk1ZB7jixKuAQXCID5jMR1qibviBQyvg2ca/Ug4a0R6b2zwKteC
         w3mSEYFgj/W05Z0h2BsSc2Il9qouMObj0x2BCaUy2vYWyGVgw9ulv8nQMq4yV7sa2wU2
         92OijM4eoL+SbLgm+Z/9IRhXwLaQX0R/T0w5Y5J6gJs/DZIDoo8XqLGJh6NaZ3B/aVro
         TmThBB4k+2DHHTwLY5l/ViEC5ZZJTDKRBlWvFKM+EPteywP8jNXUJT4Crg6eDyhP4Ym5
         UCIQ==
ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816;
        h=user-agent:in-reply-to:content-disposition:mime-version:references
         :message-id:subject:cc:to:from:date:dkim-signature
         :arc-authentication-results;
        bh=vI1rwdNtDYz0On2pzZ/tLSf+3M73hseWGO2Bt9M/LDk=;
        b=rDfTYL83z+g3EXObPNHVhdR5KzPp5k3pAqHSoRTfMick/nFMOpDtD7ZedWMh2NKJ6O
         rzHJdoSXOj1t/EHvd0U0SoNAc3nKiN+IDPqY0uHNdlaEtXA5YpUwYuTEmNT9Zgr6VFk2
         hmPC2t2SPhzzjsUldDFXiHprcIhzA6dEPnuK8OV4BGB7Lx9Fs3mbepbQihNighdVmvqV
         MFLlehUHCWn/3rybShfDRThbr6zFtrx3gXjMnFplHxk6P9M3Qkmw3e4O1okMMMaf0/9a
         Ewl5kozmHBn+OpCrpI+scjcGEMZfzhED/uJxHmRWSFnxzOIWmbEfC0UtsnVwYpBYmybV
         DaRg==
ARC-Authentication-Results: i=1; mx.google.com;
       dkim=pass header.i=@infradead.org header.s=merlin.20170209 header.b=R1YLS8+A;
       spf=pass (google.com: best guess record for domain of peterz@infradead.org designates 205.233.59.134 as permitted sender) smtp.mailfrom=peterz@infradead.org
Authentication-Results: mx.google.com;
       dkim=pass header.i=@infradead.org header.s=merlin.20170209 header.b=R1YLS8+A;
       spf=pass (google.com: best guess record for domain of peterz@infradead.org designates 205.233.59.134 as permitted sender) smtp.mailfrom=peterz@infradead.org
Date: Thu, 25 Jan 2018 19:18:52 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>,
	Tim Chen <tim.c.chen@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>
Subject: Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same
 user process
Message-ID: <20180125181852.GL2249@hirez.programming.kicks-ass.net>
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>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CALCETrVeb7g+wAP8hox9TEBBWYAFXGzOv5t2aQVrtuW4D6NQQQ@mail.gmail.com>
User-Agent: Mutt/1.9.2 (2017-12-15)
X-getmail-retrieved-from-mailbox: INBOX
X-GMAIL-THRID: =?utf-8?q?1590523944221221739?=
X-GMAIL-MSGID: =?utf-8?q?1590589493699623450?=
X-Mailing-List: linux-kernel@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>

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..