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 16:45:54 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:     Arjan van de Ven <arjan@...ux.intel.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Liran Alon <liran.alon@...cle.com>, jmattson@...gle.com,
        x86@...nel.org, dwmw@...zon.co.uk, bp@...en8.de,
        aliguori@...zon.com, thomas.lendacky@....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

On 10/01/2018 16:41, Konrad Rzeszutek Wilk wrote:
> On Wed, Jan 10, 2018 at 03:28:43PM +0100, Paolo Bonzini wrote:
>> On 10/01/2018 15:06, Arjan van de Ven wrote:
>>> On 1/10/2018 5:20 AM, Paolo Bonzini wrote:
>>>> * a simple specification that does "IBRS=1 blocks indirect branch
>>>> prediction altogether" would actually satisfy the specification just as
>>>> well, and it would be nice to know if that's what the processor actually
>>>> does.
>>>
>>> it doesn't exactly, not for all.
>>>
>>> so you really do need to write ibrs again.
>>
>> Okay, so "always set IBRS=1" does *not* protect against variant 2.  Thanks,
> 
> And what is the point of this "always set IBRS=1" then? Are there some other
> things lurking in the shadows?

The idea was that:

1) for workloads that are very kernel-heavy "always set IBRS=1" would be
faster than flipping it on kernel entry/exit,

2) skipping the IBRS=1 write on vmexit would be a nice optimization
because most vmexits come from guest ring 0.

But apparently it's a non-starter. :(

Paolo