linux-kernel - Re: [PATCH v3 0/4] KVM: Expose speculation control feature to guests

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Message-ID: <e9d538c7-45db-ee9d-ceb4-f1c2569a6d59@intel.com>
Date:   Tue, 30 Jan 2018 22:54:37 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        "Raj, Ashok" <ashok.raj@...el.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        KarimAllah Ahmed <karahmed@...zon.de>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        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>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH v3 0/4] KVM: Expose speculation control feature to guests

On 01/30/2018 04:16 PM, Paolo Bonzini wrote:
> On 30/01/2018 18:48, Raj, Ashok wrote:
>>> Certainly not every vmexit!  But doing it on every userspace vmexit and
>>> every sched_out would not be *that* bad.
>> Right.. agreed. We discussed the different scenarios that doing IBPB
>> on VMexit would help, and decided its really not required on every exit. 
>>
>> One obvious case is when there is a VMexit and return back to Qemu
>> process (witout a real context switch) do we need that to be 
>> protected from any poisoned BTB from guest?
> If the host is using retpolines, then some kind of barrier is needed.  I
> don't know if the full PRED_CMD barrier is needed, or two IBRS=1/IBRS=0
> writes back-to-back are enough.

I think the spec is pretty clear here: protection is only provided
*while* IBRS=1.  Once it goes back to 0, all bets are off.