linux-kernel - Re: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure

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Message-ID: <20180131150747.GG2521@work-vm>
Date:   Wed, 31 Jan 2018 15:07:48 +0000
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Andi Kleen <ak@...ux.intel.com>, Jim Mattson <jmattson@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Eduardo Habkost <ehabkost@...hat.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        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>,
        KVM list <kvm@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support
 infrastructure

* Paolo Bonzini (pbonzini@...hat.com) wrote:
> On 29/01/2018 22:13, Andi Kleen wrote:
> >> What happens when someone introduces a
> >> workaround tied to some other model numbers?
> > There are already many of those in the tree for other issues and features. 
> > So far you managed to survive without. Likely that will be true
> > in the future too.
> 
> "Guests have to live with processor fuckups" is actually a much better
> answer than "Hypervisors may need to revisit their practice", since at
> least it's clear where the blame lies.
> 
> Because really it's just plain luck.  It just happens that most errata
> are for functionality that is not available to a virtual machine (e.g.
> perfmon and monitor workarounds or buggy TSC deadline timer that
> hypervisors emulate anyway), that only needs a chicken bit to be set in
> the host, or the bugs are there only for old hardware that doesn't have
> virtualization (X86_BUG_F00F, X86_BUGS_SWAPGS_FENCE).
> 
> CPUID flags are guaranteed to never change---never come, never go away.
> For anything that doesn't map nicely to a CPUID flag, you cannot really
> express it.  Also if something is not architectural, you can pretty much
> assume that you cannot know it under virtualization.  f/m/s is not
> architectural; family, model and stepping mean absolutely nothing when
> running in virtualization, because the host CPU model can change under
> your feet at any time.  We force guest vendor == host vendor just
> because otherwise too much stuff breaks, but that's it.

In some ways we've been luckiest on x86; my understanding is ARM have a
similar set of architecture-specific errata and aren't really sure
how to expose this to guests either.

Dave

> Paolo
--
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK