From: David Woodhouse <dwmw2@infradead.org>
To: Paolo Bonzini <pbonzini@redhat.com>,
tglx@linutronix.de, x86@kernel.org, kvm@vger.kernel.org,
torvalds@linux-foundation.org, linux-kernel@vger.kernel.org,
arjan.van.de.ven@intel.com, dave.hansen@intel.com
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs
Date: Tue, 13 Feb 2018 10:36:38 +0000 [thread overview]
Message-ID: <1518518198.12890.48.camel@infradead.org> (raw)
In-Reply-To: <1518517262.12890.43.camel@infradead.org>
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On Tue, 2018-02-13 at 10:21 +0000, David Woodhouse wrote:
>
> > So the right logic is:
> >
> > - if the VM has IBRS_ALL, pass through the MSR when it is zero and
> > intercept writes when it is one (no writes should happen)
> >
> > - if the VM doesn't have IBRS_ALL, do as we are doing now, independent
> > of what the host spectre_v2_ibrs_all() setting is.
>
> We end up having to turn IBRS on again on vmexit then, taking care that
> no conditional branch can go round it. So that becomes an
> *unconditional* wrmsr or lfence in the vmexit path. We really don't
> want that.
Note that being able to keep it simple in KVM was basically what made
the difference between me tolerating IBRS_ALL as Intel currently define
it, and throwing my toys out of the pram (as I had done in the first
iterations of this patch).
If we *can't* keep it nice and simple in KVM, then I think we *really*
want Intel to make the hardware bit a no-op. If not for all CPUs with
the IBRS_ALL bit, then for all *future* CPUs and they can define a new
IBRS_ALL_UNCONDITIONAL bit. Which is the only one we'd ever care about.
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next prev parent reply other threads:[~2018-02-13 10:36 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-12 15:27 [PATCH 1/2] x86/speculation: Correct Speculation Control microcode blacklist again David Woodhouse
2018-02-12 15:27 ` [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs David Woodhouse
2018-02-13 7:47 ` Ingo Molnar
2018-02-13 8:12 ` David Woodhouse
2018-02-13 8:02 ` Paolo Bonzini
2018-02-13 8:15 ` David Woodhouse
2018-02-13 9:58 ` Paolo Bonzini
2018-02-13 10:21 ` David Woodhouse
2018-02-13 10:36 ` David Woodhouse [this message]
2018-02-13 10:41 ` Paolo Bonzini
2018-02-13 10:53 ` David Woodhouse
2018-02-13 10:55 ` Paolo Bonzini
2018-02-16 9:58 ` David Woodhouse
2018-02-16 10:08 ` Paolo Bonzini
2018-02-16 10:21 ` David Woodhouse
2018-02-16 11:04 ` Paolo Bonzini
2018-02-16 12:10 ` David Woodhouse
2018-02-15 15:21 ` Pavel Machek
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