linux-kernel - Re: [V2 PATCH 4/6] KVM: selftests: x86: Add helpers to execute VMs with private memory

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Message-ID: <Y8cmfjRIRp2EphTa@google.com>
Date:   Tue, 17 Jan 2023 22:51:42 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vishal Annapurve <vannapurve@...gle.com>
Cc:     x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, pbonzini@...hat.com,
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Subject: Re: [V2 PATCH 4/6] KVM: selftests: x86: Add helpers to execute VMs
 with private memory

On Mon, Dec 05, 2022, Vishal Annapurve wrote:
> +void vcpu_run_and_handle_mapgpa(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
> +{
> +	/*
> +	 * Loop until the guest exits with any reason other than
> +	 * KVM_HC_MAP_GPA_RANGE hypercall.
> +	 */
> +
> +	while (true) {
> +		vcpu_run(vcpu);
> +
> +		if ((vcpu->run->exit_reason == KVM_EXIT_HYPERCALL) &&
> +			(vcpu->run->hypercall.nr == KVM_HC_MAP_GPA_RANGE)) {

I get what you're trying to do, and I completely agree that we need better helpers
and/or macros to reduce this type of boilerplate, but adding a one-off helper like
this is going to be a net negative overall.  This helper services exactly one use
case, and also obfuscates what a test does.

In other words, this is yet another thing that needs broad, generic support
(_vcpu_run() is a very special case).  E.g. something like this to make it easy
for tests to run a guest and handle ucalls plus specific exits (just a strawman,
I think we can do better for handling ucalls).

#define vcpu_run_loop(vcpu, handlers, ucalls)				\
do {									\
	uint32_t __exit;						\
	int __r = 0;							\
									\
	while (!r)  {							\
		vcpu_run(vcpu);						\
									\
		__exit = vcpu->run->exit_reason;			\
									\
		if (__exit < ARRAY_SIZE(handlers) && handlers[__exit])	\
			__r = handlers[__exit](vcpu);			\	
		else if (__exit == KVM_EXIT_IO && ucalls)		\
			__r = handle_exit_ucall(vcpu, ucalls,		\
						ARRAY_SIZE(ucalls));	\
		else							\
			TEST_FAIL(...)					\
	}								\
} while (0)


For this series, I think it makes sense to just open code yet another test.  It
really doesn't end up being much code, which is partly why we haven't added
helpers :-/