linux-kernel - Re: [PATCH v2 05/49] KVM: selftests: Assert that the @cpuid passed to get_cpuid

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Message-ID: <Zz-ClqMVuOrFlIZK@google.com>
Date: Thu, 21 Nov 2024 10:57:26 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Maxim Levitsky <mlevitsk@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Hou Wenlong <houwenlong.hwl@...group.com>, 
	Kechen Lu <kechenl@...dia.com>, Oliver Upton <oliver.upton@...ux.dev>, 
	Binbin Wu <binbin.wu@...ux.intel.com>, Yang Weijiang <weijiang.yang@...el.com>, 
	Robert Hoo <robert.hoo.linux@...il.com>
Subject: Re: [PATCH v2 05/49] KVM: selftests: Assert that the @cpuid passed to
 get_cpuid_entry() is non-NULL

On Wed, Jul 24, 2024, Maxim Levitsky wrote:
> On Mon, 2024-07-08 at 19:33 +0000, Sean Christopherson wrote:
> > On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> > > On Fri, 2024-05-17 at 10:38 -0700, Sean Christopherson wrote:
> > > > Add a sanity check in get_cpuid_entry() to provide a friendlier error than
> > > > a segfault when a test developer tries to use a vCPU CPUID helper on a
> > > > barebones vCPU.
> > > > 
> > > > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > > > ---
> > > >  tools/testing/selftests/kvm/lib/x86_64/processor.c | 2 ++
> > > >  1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> > > > index c664e446136b..f0f3434d767e 100644
> > > > --- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
> > > > +++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> > > > @@ -1141,6 +1141,8 @@ const struct kvm_cpuid_entry2 *get_cpuid_entry(const struct kvm_cpuid2 *cpuid,
> > > >  {
> > > >  	int i;
> > > >  
> > > > +	TEST_ASSERT(cpuid, "Must do vcpu_init_cpuid() first (or equivalent)");
> > > > +
> > > >  	for (i = 0; i < cpuid->nent; i++) {
> > > >  		if (cpuid->entries[i].function == function &&
> > > >  		    cpuid->entries[i].index == index)
> > > 
> > > Hi,
> > > 
> > > Maybe it is better to do this assert in __vcpu_get_cpuid_entry() because the
> > > assert might confuse the reader, since it just tests for NULL but when it
> > > fails, it complains that you need to call some function.
> > 
> > IIRC, I originally added the assert in __vcpu_get_cpuid_entry(), but I didn't
> > like leaving get_cpuid_entry() unprotected.  What if I add an assert in both?
> > E.g. have __vcpu_get_cpuid_entry() assert with the (hopefully) hepful message,
> > and have get_cpuid_entry() do a simple TEST_ASSERT_NE()?
> > 
> 
> This looks like a great idea.

Circling back to this, I actually like your initial suggestion better.  Asserting
in get_cpuid_entry() is unnecessary paranoia, e.g. it's roughly equivalent to
asserting that any and all pointers are non-NULL.   The __vcpu_get_cpuid_entry()
assert though makes a lot more sense, because it's not all that obvious that
vcpu->cpuid is (usually) initialized elsewhere.