linux-kernel - Re: [PATCH v2] x86/ibpb: Skip IBPB when we switch back to same user process

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Message-ID: <20180128095609.rfcnv3itu3clrkru@gmail.com>
Date:   Sun, 28 Jan 2018 10:56:09 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Laura Abbott <labbott@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, rkrcmar@...hat.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Lendacky <thomas.lendacky@....com>, x86@...nel.org
Subject: Re: [PATCH v2] x86/ibpb: Skip IBPB when we switch back to same user
 process


* Tim Chen <tim.c.chen@...ux.intel.com> wrote:

> Thanks to the reviewers and Andy Lutomirski for the suggestion of
> using ctx_id which got rid of the problem of mm pointer recycling.
> Here's an update of this patch based on Andy's suggestion.
> 
> We could switch to a kernel idle thread and then back to the original
> process such as:
> process A -> idle -> process A
> 
> In such scenario, we do not have to do IBPB here even though the process is
> non-dumpable, as we are switching back to the same process after
> an hiatus.
> 
> We track the last mm user context id before we switch to init_mm by calling
> leave_mm when tlb_defer_switch_to_init_mm returns false (pcid available).
> 
> The cost is to have an extra u64 mm context id to track the last mm we were using before
> switching to the init_mm used by idle.  Avoiding the extra IBPB
> is probably worth the extra memory for this common scenario.
> 
> For those cases where tlb_defer_switch_to_init_mm returns true (non pcid),
> lazy tlb will defer switch to init_mm, so we will not be changing
> the mm for the process A -> idle -> process A switch. So
> IBPB will be skipped for this case.
> 
> v2:
> 1. Save last user context id instead of last user mm to avoid the problem of recycled mm
> 
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
>  arch/x86/include/asm/tlbflush.h |  2 ++
>  arch/x86/mm/tlb.c               | 23 ++++++++++++++++-------
>  2 files changed, 18 insertions(+), 7 deletions(-)

What tree is this patch against? It doesn't apply to linus's latest, nor to 
tip:master.

Thanks,

	Ingo