linux-kernel - Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs

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Message-ID: <27c85759-e662-d281-f8a0-0a80ca8ee18f@redhat.com>
Date:   Tue, 13 Feb 2018 10:58:00 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Woodhouse <dwmw2@...radead.org>, tglx@...utronix.de,
        x86@...nel.org, kvm@...r.kernel.org, torvalds@...ux-foundation.org,
        linux-kernel@...r.kernel.org, arjan.van.de.ven@...el.com,
        dave.hansen@...el.com
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
 CPUs

On 13/02/2018 09:15, David Woodhouse wrote:
>>>  
>>> -		if (!data)
>>> +		if (!data && !spectre_v2_ibrs_all())
>>>  			break;
>> This should check the value of IBRS_ALL in the VM, not in the host.
> No, it's host we want. If IBRS_ALL is set in the host, we set the
> actual hardware MSR once at boot time and never touch it again. The
> SPEC_CTRL MSR we expose to guests is purely a no-op fiction.
> 
> If spectre_v2_ibrs_all() is true then KVM should *never* actually pass
> through or touch the real MSR.

That would be nice but unfortunately it's not possible. :(

The VM might actually not have IBRS_ALL, as usual the reason is
migration compatibility.  In that case, that no-op fiction would be very
slow because the VM will actually do a lot of SPEC_CTRL writes.

So the right logic is:

- if the VM has IBRS_ALL, pass through the MSR when it is zero and
intercept writes when it is one (no writes should happen)

- if the VM doesn't have IBRS_ALL, do as we are doing now, independent
of what the host spectre_v2_ibrs_all() setting is.

Paolo