linux-kernel - Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs

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Date:   Thu, 15 Feb 2018 16:21:39 +0100
From:   Pavel Machek <pavel@....cz>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Woodhouse <dwmw@...zon.co.uk>, tglx@...utronix.de,
        x86@...nel.org, kvm@...r.kernel.org, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, arjan.van.de.ven@...el.com,
        dave.hansen@...el.com
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
 CPUs

On Tue 2018-02-13 09:02:25, Paolo Bonzini wrote:
> On 12/02/2018 16:27, David Woodhouse wrote:
> > The original IBRS hack in microcode is horribly slow. For the next
> > generation of CPUs, as a stopgap until we get a proper fix, Intel
> > promise an "Enhanced IBRS" which will be fast.
> > 
> > The assumption is that predictions in the BTB/RSB will be tagged with
> > the VMX mode and ring that they were learned in, and thus the CPU will
> > avoid consuming unsafe predictions without a performance penalty.
> > 
> > Intel's documentation says that it is still required to set the IBRS bit
> > in the SPEC_CTRL MSR and ensure that it remains set.
> > 
> > Cope with this by trapping and emulating *all* access to SPEC_CTRL from
> > KVM guests when the IBRS_ALL feature is present, so it can never be
> > turned off. Guests who see IBRS_ALL should never do anything except
> > turn it on at boot anyway. And if they didn't know about IBRS_ALL and
> > they keep frobbing IBRS on every kernel entry/exit... well the vmexit
> > for a no-op is probably going to be faster than they were expecting
> > anyway, so they'll live.
> > 
> > Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> > Acked-by: Arjan van de Ven <arjan.van.de.ven@...el.com>
> > ---
> >  arch/x86/include/asm/nospec-branch.h |  9 ++++++++-
> >  arch/x86/kernel/cpu/bugs.c           | 16 ++++++++++++++--
> >  arch/x86/kvm/vmx.c                   | 17 ++++++++++-------
> >  3 files changed, 32 insertions(+), 10 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> > index 788c4da..524bb86 100644
> > --- a/arch/x86/include/asm/nospec-branch.h
> > +++ b/arch/x86/include/asm/nospec-branch.h
> > @@ -140,9 +140,16 @@ enum spectre_v2_mitigation {
> >  	SPECTRE_V2_RETPOLINE_MINIMAL_AMD,
> >  	SPECTRE_V2_RETPOLINE_GENERIC,
> >  	SPECTRE_V2_RETPOLINE_AMD,
> > -	SPECTRE_V2_IBRS,
> > +	SPECTRE_V2_IBRS_ALL,
> >  };
> >  
> > +extern enum spectre_v2_mitigation spectre_v2_enabled;
> > +
> > +static inline bool spectre_v2_ibrs_all(void)
> > +{
> > +	return spectre_v2_enabled == SPECTRE_V2_IBRS_ALL;
> > +}
> > +
> >  extern char __indirect_thunk_start[];
> >  extern char __indirect_thunk_end[];
> >  
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index debcdda..047538a 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -88,12 +88,13 @@ static const char *spectre_v2_strings[] = {
> >  	[SPECTRE_V2_RETPOLINE_MINIMAL_AMD]	= "Vulnerable: Minimal AMD ASM retpoline",
> >  	[SPECTRE_V2_RETPOLINE_GENERIC]		= "Mitigation: Full generic retpoline",
> >  	[SPECTRE_V2_RETPOLINE_AMD]		= "Mitigation: Full AMD retpoline",
> > +	[SPECTRE_V2_IBRS_ALL]			= "Mitigation: Enhanced IBRS",
> >  };

Hmm. Probably not just your problem but these should really get
documentation somewhere -- and adding another one should be treated
like changing the ABI.

How is poor userland expected to do anything inteligent with that
file?

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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