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[Jon Masters <jcm@redhat.com>]

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From: Jon Masters <jcm@redhat.com>
To: speck for Thomas Gleixner <speck@linutronix.de>
Subject: Re: [patch V6 12/14] MDS basics 12

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On 3/1/19 4:47 PM, speck for Thomas Gleixner wrote:

> Subject: [patch V6 12/14] x86/speculation/mds: Add mitigation mode VMWERV
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> In virtualized environments it can happen that the host has the microcode
> update which utilizes the VERW instruction to clear CPU buffers, but the
> hypervisor is not yet updated to expose the X86_FEATURE_MD_CLEAR CPUID bit
> to guests.
> 
> Introduce an internal mitigation mode VWWERV which enables the invocation
> of the CPU buffer clearing even if X86_FEATURE_MD_CLEAR is not set. If the
> system has no updated microcode this results in a pointless execution of
> the VERW instruction wasting a few CPU cycles. If the microcode is updated,
> but not exposed to a guest then the CPU buffers will be cleared.
> 
> That said: Virtual Machines Will Eventually Receive Vaccine

The effect of this patch, currently, is that a (bare metal) machine
without updated ucode will print the following:

[    1.576602] MDS: Vulnerable: Clear CPU buffers attempted, no microcode

The intention of the patch is to say "hey, you might be on a VM, so
we'll try anyway in case we didn't get told you had MD_CLEAR". But the
effect on bare metal might be ambiguous. It's reasonable (for someone
else) to assume we might be using a software sequence to try flushing.

Perhaps the wording should convey something like:

"MDS: Vulnerable: Clear CPU buffers may not work, no microcode"

Jon.

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