linux-kernel - Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC

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Date:   Thu, 1 Feb 2018 09:09:01 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Jim Mattson <jmattson@...gle.com>
Cc:     KarimAllah Ahmed <karahmed@...zon.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to
 MSR_IA32_SPEC_CTRL

On 31/01/2018 16:59, David Woodhouse wrote:
> 
> 
> On Wed, 2018-01-31 at 13:53 -0800, Jim Mattson wrote:
>> On Wed, Jan 31, 2018 at 1:42 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>>> Can we just say it sucks to be L2 too? :)  Because in the end as long as
>>> no one ever writes to spec_ctrl, everybody is happy.
>>
>> Unfortunately, quite a few OS vendors shipped IBRS-based mitigations
>> earlier this month. (Has Redhat stopped writing to IA32_SPEC_CTRL yet?
>> :-)
>>
>> And in the long run, everyone is going to set IA32_SPEC_CTRL.IBRS=1 on
>> CPUs with IA32_ARCH_CAPABILITIES.IBRS_ALL.
> 
> 
> I'm actually working on IBRS_ALL at the moment.
> 
> I was tempted to *not* let the guests turn it off. Expose SPEC_CTRL but
> just make it a no-op.

That would be very slow.

> Or if that really doesn't fly, perhaps with IBRS_ALL we should invert
> the logic. Set IBRS to 1 on vCPU reset, and only if it's set to *zero*
> do we pass through the MSR and set the save_spec_ctrl_on_exit flag.

... but something like that would be a good idea.  Even if IBRS to 0 on
vCPU reset, only pass it through once it's set to zero.  The first
IBRS=1 write would not enable pass through, and would not set the
save_spec_ctrl_on_exit flag.  In fact it need not even be conditional on
IBRS_ALL.

Paolo

> But let's get the code for *current* hardware done first...
>