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

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Message-ID: <65578664-e3ec-f894-4e94-ff9fe6d7d6b3@redhat.com>
Date:   Wed, 10 Jan 2018 17:27:47 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Liran Alon <liran.alon@...cle.com>, dwmw@...zon.co.uk
Cc:     konrad.wilk@...cle.com, jmattson@...gle.com, x86@...nel.org,
        bp@...en8.de, nadav.amit@...il.com, thomas.lendacky@....com,
        aliguori@...zon.com, arjan@...ux.intel.com, rkrcmar@...hat.com,
        linux-kernel@...r.kernel.org, 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

I can answer (2) only.

On 10/01/2018 17:19, Liran Alon wrote:
> (2) On VMExit, Intel recommends to always save guest SPEC_CTRL value,
> set IBRS to 1 (even if it is already set by guest) and stuff RSB. What
> exactly does this write of 1 to IBRS do?
> * a) Does it keep all currently existing BTB/BHB entries created by
> less-privileged prediction-mode and marks them as were created in
> less-privileged prediction-mode such that they won't be used in current
> prediction-mode?
> * b) Or does it, as Paolo has mentioned multiple times, disables the
> branch predictor to never consult the BTB/BHB at all as long as IBRS=1?
> If (b) is true, why is setting IBRS=1 better than just issue an IBPB that clears all entries? At least in that case the > host kernel could still benefict branch prediction performance boost.

Arjan said (b) is not true on all processor generations.  But even if it
were true, setting IBRS=1 is much, much faster than IBPB.

> 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.
While I agree with David that "we have other things to work on for now
before we support hypothetical future hardware", I'd like to make sure
that Intel gets it right...

Paolo