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

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Date:   Tue, 30 Jan 2018 19:16:38 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "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>,
        Dave Hansen <dave.hansen@...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 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.

If the host is using IBRS, then writing IBRS=1 at vmexit has established
a barrier from the less privileged VMX guest environment.

Paolo

> If Qemu is protected by !dumpable/retpoline that should give that gaurantee.
> We do VM->VM IBPB at vmload() time that should provide that gaurantee.