From: Arjan van de Ven <arjan@linux.intel.com>
To: Paolo Bonzini <pbonzini@redhat.com>, Liran Alon <liran.alon@oracle.com>
Cc: jmattson@google.com, dwmw@amazon.co.uk, bp@alien8.de,
thomas.lendacky@amd.com, aliguori@amazon.com,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH 6/7] x86/svm: Set IBPB when running a different VCPU
Date: Tue, 9 Jan 2018 06:30:09 -0800 [thread overview]
Message-ID: <0c5e90b5-e1f2-1ae1-40d1-d77575f580e9@linux.intel.com> (raw)
In-Reply-To: <713b5681-a6eb-7a7b-620f-282bd67eb745@redhat.com>
On 1/9/2018 3:41 AM, Paolo Bonzini wrote:
> The above ("IBRS simply disables the indirect branch predictor") was my
> take-away message from private discussion with Intel. My guess is that
> the vendors are just handwaving a spec that doesn't match what they have
> implemented, because honestly a microcode update is unlikely to do much
> more than an old-fashioned chicken bit. Maybe on Skylake it does
> though, since the performance characteristics of IBRS are so different
> from previous processors. Let's ask Arjan who might have more
> information about it, and hope he actually can disclose it...
IBRS will ensure that, when set after the ring transition, no earlier
branch prediction data is used for indirect branches while IBRS is set
(this is a english summary of two pages of technical spec so it lacks
the language lawyer precision)
because of this promise, the implementation tends to be impactful
and it is very strongly recommended that retpoline is used instead of IBRS.
(with all the caveats already on lkml)
the IBPB is different, this is a covenient thing for switching between VM guests etc
next prev parent reply other threads:[~2018-01-09 14:30 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-09 11:31 [PATCH 6/7] x86/svm: Set IBPB when running a different VCPU Liran Alon
2018-01-09 11:41 ` Paolo Bonzini
2018-01-09 14:30 ` Arjan van de Ven [this message]
-- strict thread matches above, loose matches on Subject: below --
2018-01-09 16:01 Liran Alon
2018-01-09 15:33 Liran Alon
2018-01-09 15:56 ` Arjan van de Ven
2018-01-09 15:00 Liran Alon
2018-01-09 15:19 ` Arjan van de Ven
2018-01-09 16:17 ` Paolo Bonzini
2018-01-09 16:23 ` Arjan van de Ven
2018-01-09 16:49 ` Paolo Bonzini
2018-01-09 20:39 ` Konrad Rzeszutek Wilk
2018-01-09 20:47 ` Konrad Rzeszutek Wilk
2018-01-09 20:57 ` Jim Mattson
2018-01-09 21:11 ` Konrad Rzeszutek Wilk
2018-01-09 21:19 ` Jim Mattson
2018-01-09 21:42 ` Paolo Bonzini
2018-01-09 21:59 ` Jim Mattson
2018-01-09 21:56 ` Paolo Bonzini
2018-01-08 18:08 [PATCH 0/7] KVM: x86: expose CVE-2017-5715 ("Spectre variant 2") mitigations to guest Paolo Bonzini
2018-01-08 18:08 ` [PATCH 6/7] x86/svm: Set IBPB when running a different VCPU Paolo Bonzini
2018-01-08 20:00 ` Liran Alon
2018-01-09 11:07 ` Paolo Bonzini
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