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Subject: Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same
 user process
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Tim Chen <tim.c.chen@linux.intel.com>,
        linux-kernel@vger.kernel.org,
        KarimAllah Ahmed <karahmed@amazon.de>,
        Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        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>, rkrcmar@redhat.com,
        Thomas Gleixner <tglx@linutronix.de>,
        Tom Lendacky <thomas.lendacky@amd.com>, x86@kernel.org
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From:   Arjan van de Ven <arjan@linux.intel.com>
Message-ID: <565574d1-c0a6-684d-a387-12f2cdaf01de@linux.intel.com>
Date:   Thu, 25 Jan 2018 09:24:53 -0800
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> The idea is simple, do what we do for virt. Don't send IPI's to CPUs
> that don't need them (in virt's case because the vCPU isn't running, in
> our case because we're not in fact running a user process), but mark the
> CPU as having needed a TLB flush.

I am really uncomfortable with that idea.
You really can't run code safely on a cpu where the TLBs in the CPU are invalid
or where a CPU that does (partial) page walks would install invalid PTEs either
through actual or through speculative execution.

(in the virt case there's a cheat, since the code is not actually running
there isn't a cpu with TLBs live. You can't do that same cheat for this case)