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Message-ID: <1517171968.6624.80.camel@infradead.org>
Subject: Re: [PATCH] x86: vmx: Allow direct access to MSR_IA32_SPEC_CTRL
From:   David Woodhouse <dwmw2@infradead.org>
To:     Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        KarimAllah Ahmed <karahmed@amazon.de>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Asit Mallick <asit.k.mallick@intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dan Williams <dan.j.williams@intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Ashok Raj <ashok.raj@intel.com>, daniel.kiper@oracle.com
In-Reply-To: <4DCAF18F-C86A-4CBC-A9CC-CC01BF63313F@oracle.com>
References: <1517167750-23485-1-git-send-email-karahmed@amazon.de>
         <4DCAF18F-C86A-4CBC-A9CC-CC01BF63313F@oracle.com>
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Date:   Sun, 28 Jan 2018 20:39:28 +0000
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On Sun, 2018-01-28 at 15:21 -0500, Konrad Rzeszutek Wilk wrote:
> >To avoid the overhead of atomically saving and restoring the MSR_IA32_SP=
EC_CTRL
> >for guests that do not actually use the MSR, only add_atomic_switch_msr =
when a
> >non-zero is written to it.
>=20
>=20
> We tried this and found that it was about 3% slower that doing the
> old way of rdmsr and wrmsr.
>=20
> But that was also with the host doing IBRS=C2=A0 as well.

The common case will be that neither host nor guest are doing IBRS.
Until the guest touches the MSR we do absolutely *nothing* with it,
which is definitely the fastest option.
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