linux-kernel - Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and MSR_IA32_PRED

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Date:   Wed, 10 Jan 2018 18:16:09 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Liran Alon <liran.alon@...cle.com>, dwmw@...zon.co.uk,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        the arch/x86 maintainers <x86@...nel.org>, bp@...en8.de,
        Nadav Amit <nadav.amit@...il.com>,
        Tom Lendacky <thomas.lendacky@....com>, aliguori@...zon.com,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH 3/8] kvm: vmx: pass MSR_IA32_SPEC_CTRL and
 MSR_IA32_PRED_CMD down to the guest

On 10/01/2018 18:14, Jim Mattson wrote:
>>> If (a) is true, does "IBRS ALL THE TIME" usage is basically a CPU
>>> change to just create all BTB/BHB entries to be tagged with
>>> prediction-mode at creation-time and that tag to be compared to current
>>> prediction-mode when CPU attempts to use BTB/BHB?
>>
>> I hope so, and I hope said prediction mode includes PCID/VPID too.
>
> Branch prediction entries should probably be tagged with PCID, VPID,
> EP4TA, and thread ID...the same things used to tag TLB contexts.

But if so, I don't see the need for IBPB.

Paolo