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

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Date:   Fri, 26 Jun 2020 13:37:50 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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 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).

Thanks,

-- 
Peter Xu