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

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Date:   Tue, 30 Jan 2018 18:36:20 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        KarimAllah Ahmed <karahmed@...zon.de>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     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>,
        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 04:00, David Woodhouse wrote:
> I believe Ashok sent you a change which made us do IBPB on *every*
> vmexit; I don't think we need that. It's currently done in vcpu_load()
> which means we'll definitely have done it between running one vCPU and
> the next, and when vCPUs are pinned we basically never need to do it.
> 
> We know that VMM (e.g. qemu) userspace could be vulnerable to attacks
> from guest ring 3, because there is no flush between the vmexit and the
> host kernel "returning" to the userspace thread. Doing a full IBPB on
> *every* vmexit would protect from that, but it's overkill. If that's
> the reason, let's come up with something better.

Certainly not every vmexit!  But doing it on every userspace vmexit and
every sched_out would not be *that* bad.

We try really hard to avoid userspace vmexits for everything remotely
critical to performance (the main exception that's left is the PMTIMER
I/O port, that Windows likes to access quite a lot), so they shouldn't
happen that often.

Paolo