linux-kernel - Re: [PATCH 1/2] KVM: X86: Move ignore

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Date:   Fri, 26 Jun 2020 08:56:57 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH 1/2] KVM: X86: Move ignore_msrs handling upper the stack

On Thu, Jun 25, 2020 at 08:44:16PM +0200, Paolo Bonzini wrote:
> On 25/06/20 18:25, Sean Christopherson wrote:
> > I get the "what" of the change, and even the "why" to some extent, but I
> > dislike the idea of supporting/encouraging blind reads/writes to MSRs.
> > Blind writes are just asking for problems, and suppressing warnings on reads
> > is almost guaranteed to be suppressing a KVM bug.
> 
> Right, that's why this patch does not just suppress warnings: it adds a
> different return value to detect the case.
> 
> > TSC_CTRL aside, if we insist on pointing a gun at our foot at some point,
> > this should be a dedicated flavor of MSR access, e.g. msr_data.kvm_initiated,
> > so that it at least requires intentionally loading the gun.
> 
> With this patch, __kvm_get_msr does not know about ignore_msrs at all,
> that seems to be strictly an improvement; do you agree with that?

Not really?  It's solving a problem that doesn't exist in the current code
base (assuming TSC_CTRL is fixed), and IMO solving it in an ugly fashion.

I would much prefer that, _if_ we want to support blind KVM-internal MSR
accesses, we end up with code like:

	if (msr_info->kvm_internal) {
		return 1;
	} else if (!ignore_msrs) {
		vcpu_debug_ratelimited(vcpu, "unhandled wrmsr: 0x%x data 0x%llx\n",
			    msr, data);
		return 1;
	} else {
		if (report_ignored_msrs)
			vcpu_unimpl(vcpu,
				"ignored wrmsr: 0x%x data 0x%llx\n",
				msr, data);
		break;
	}

But I'm still not convinced that there is a legimiate scenario for setting
kvm_internal=true.

> What would you think about adding warn_unused_result to __kvm_get_msr?

I guess I wouldn't object to it, but that seems like an orthogonal issue.