linux-kernel - Re: [PATCH v2] KVM: allow KVM_BUG/KVM_BUG

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Message-ID: <ZH39H0gpNX4ak6yM@google.com>
Date:   Mon, 5 Jun 2023 08:19:59 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Michal Luczaj <mhal@...x.co>
Cc:     dmatlack@...gle.com, mizhang@...gle.com, isaku.yamahata@...il.com,
        pbonzini@...hat.com, Wei Wang <wei.w.wang@...el.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: allow KVM_BUG/KVM_BUG_ON to handle 64-bit cond

On Mon, Jun 05, 2023, Michal Luczaj wrote:
> On 6/2/23 18:56, Sean Christopherson wrote:
> > On Fri, Jun 02, 2023, Michal Luczaj wrote:
> >> I guess this makes the !! in kvm_vm_ioctl_create_vcpu() unnecessary:
> >>
> >> KVM_BUG_ON(!!xa_store(&kvm->vcpu_array, vcpu->vcpu_idx, vcpu, 0)...
> > 
> > Ya, I saw that, which in addition to Wei's ping, is what reminded me that the
> > KVM_BUG_ON() fix hadn't been merged.
> > 
> >> Is it worth a patch (perhaps along with chopping off !! in
> >> kvm_msr_allowed() and few other places)?
> > 
> > Yes, I think so.
> 
> OK, so xa_store() aside[*], I see some bool-to-bools:
> 
> arch/x86/kvm/x86.c:
> 	kvm_msr_allowed():allowed = !!test_bit(index - start, bitmap);
> arch/x86/kvm/hyperv.c:
> 	kvm_hv_hypercall():hc.rep = !!(hc.rep_cnt || hc.rep_idx);
> arch/x86/kvm/mmu/mmu.c:
> 	update_pkru_bitmask():
> 		pkey_bits = !!check_pkey;
> 		pkey_bits |= (!!check_write) << 1;
> arch/x86/kvm/svm/svm.c:
> 	msr_write_intercepted():return !!test_bit(bit_write,  &tmp);
> 	svm_vcpu_after_set_cpuid():
> 		2x set_msr_interception...
> tools/testing/selftests/kvm/x86_64/vmx_exception_with_invalid_guest_state.c:
> 	set_or_clear_invalid_guest_state():sregs.tr.unusable = !!set;
> 
> But perhaps this is a matter of style and those were meant to be this kind-of
> explicit?

I doubt it, I'm guessing most cases are due to the author being overzealous for
one reason or another, e.g. I suspect the test_bit() ones are due to the original
author incorrectly assuming test_bit() returned an unsigned long, i.e. the bit,
as opposed to the bool.

If you want to clean these up, I'd say "fix" the test_bit() cases, but leave the
others alone.  The test_bit() ones are clearly redundant, and IMO can be actively
due to implying test_bit() returns something other than a bool.

> [*] https://lore.kernel.org/kvm/20230605114852.288964-1-mhal@rbox.co/