linux-kernel - Re: [RFC PATCH 5/9] x86/bugs: Use Virtual MSRs to request hardware mitigations

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Date:   Wed, 14 Dec 2022 20:18:17 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Zhang Chen <chen.zhang@...el.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Chao Gao <chao.gao@...el.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH 5/9] x86/bugs: Use Virtual MSRs to request hardware
 mitigations

On Sun, Dec 11, 2022, Zhang Chen wrote:
> From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> 
> Guests that have different family/model than the host may not be aware
> of hardware mitigations(such as RRSBA_DIS_S) available on host. This is
> particularly true when guests migrate. To solve this problem Intel
> processors have added a virtual MSR interface

Is there any actual "processor" support here?  To me, this looks like Intel is
foisting a paravirt interface on KVM and other hypervisors without collaborating
with said hypervisors' developers and maintainers.

I get that some of the mitigations are vendor specific, but things like RETPOLINE
aren't vendor specific.  I haven't followed all of the mitigation stuff very
closely, but I wouldn't be surprised if there are mitigations now or in the future
that are common across architectures, e.g. arm64 and x86-64.  Intel doing its own
thing means AMD and arm64 will likely follow suit, and suddenly KVM is supporting
multiple paravirt interfaces for very similar things, without having any control
over the APIs.  That's all kinds of backwards.

And having to wait for Intel to roll out new documentation when software inevitably
comes up with some clever new mitigation doesn't exactly fill my heart with joy.