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

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Date:   Fri, 26 Jun 2020 10:46:57 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...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 Fri, Jun 26, 2020 at 01:37:50PM -0400, Peter Xu wrote:
> On Fri, Jun 26, 2020 at 08:56:57AM -0700, Sean Christopherson wrote:
> > 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.
> 
> Actually this really looks like my initial version when I was discussing this
> with Paolo before this version, but Paolo suggested what I implemented last.  I
> think I agree with Paolo that it's an improvement to have a way to get/set real
> msr value so that we don't need to further think about effects being taken with
> the two tricky msr knobs (report_ignored_msrs, ignore_msrs).  These knobs are
> even trickier to me when they're hidden deep, because they are not easily
> expected when seeing the name of the functions (e.g. __kvm_set_msr, rather than
> __kvm_set_msr_retval_fixed).

My argument is that it's a KVM bug if we ever encounter do the wrong thing
based on a KVM-internal MSR access.  The proposed change would actually make
it _harder_ to find the bug that prompted this patch, as the bogus
__kvm_get_msr() in kvm_cpuid() would silently fail.

If anything, I would argue for something like:

	if (WARN_ON_ONCE(msr_info->kvm_internal)) {
		return 1;
	} else if (!ignore_msrs) {
		...
	} else {
		...
	}

I.e. KVM-internal accesses should always pre-validate the existence of the
MSR, if not the validity of the MSR from the guest's perspective.