linux-kernel - Re: [PATCH V8 1/2] KVM: selftests: Add x86

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Date: Mon, 10 Jun 2024 17:21:59 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Reinette Chatre <reinette.chatre@...el.com>
Cc: isaku.yamahata@...el.com, pbonzini@...hat.com, erdemaktas@...gle.com, 
	vkuznets@...hat.com, vannapurve@...gle.com, jmattson@...gle.com, 
	mlevitsk@...hat.com, xiaoyao.li@...el.com, chao.gao@...el.com, 
	rick.p.edgecombe@...el.com, yuan.yao@...el.com, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH V8 1/2] KVM: selftests: Add x86_64 guest udelay() utility

On Mon, Jun 10, 2024, Reinette Chatre wrote:
> diff --git a/tools/testing/selftests/kvm/include/x86_64/processor.h b/tools/testing/selftests/kvm/include/x86_64/processor.h
> index 8eb57de0b587..b473f210ba6c 100644
> --- a/tools/testing/selftests/kvm/include/x86_64/processor.h
> +++ b/tools/testing/selftests/kvm/include/x86_64/processor.h
> @@ -23,6 +23,7 @@
>  
>  extern bool host_cpu_is_intel;
>  extern bool host_cpu_is_amd;
> +extern unsigned int tsc_khz;
>  
>  /* Forced emulation prefix, used to invoke the emulator unconditionally. */
>  #define KVM_FEP "ud2; .byte 'k', 'v', 'm';"
> @@ -815,6 +816,20 @@ static inline void cpu_relax(void)
>  	asm volatile("rep; nop" ::: "memory");
>  }
>  
> +static inline void udelay(unsigned long usec)

uint64_t instead of unsigned long?  Practically speaking it doesn't change anything,
but I don't see any reason to mix "unsigned long" and "uint64_t", e.g. the max
delay isn't a property of the address space.

> +{
> +	unsigned long cycles = tsc_khz / 1000 * usec;
> +	uint64_t start, now;
> +
> +	start = rdtsc();
> +	for (;;) {
> +		now = rdtsc();
> +		if (now - start >= cycles)
> +			break;
> +		cpu_relax();
> +	}
> +}
> +
>  #define ud2()			\
>  	__asm__ __volatile__(	\
>  		"ud2\n"	\
> diff --git a/tools/testing/selftests/kvm/lib/x86_64/processor.c b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> index c664e446136b..ff579674032f 100644
> --- a/tools/testing/selftests/kvm/lib/x86_64/processor.c
> +++ b/tools/testing/selftests/kvm/lib/x86_64/processor.c
> @@ -25,6 +25,7 @@ vm_vaddr_t exception_handlers;
>  bool host_cpu_is_amd;
>  bool host_cpu_is_intel;
>  bool is_forced_emulation_enabled;
> +unsigned int tsc_khz;

Slight preference for uint32_t, mostly because KVM stores its version as a u32.

>  static void regs_dump(FILE *stream, struct kvm_regs *regs, uint8_t indent)
>  {
> @@ -616,6 +617,8 @@ void assert_on_unhandled_exception(struct kvm_vcpu *vcpu)
>  
>  void kvm_arch_vm_post_create(struct kvm_vm *vm)
>  {
> +	int r;
> +
>  	vm_create_irqchip(vm);
>  	vm_init_descriptor_tables(vm);
>  
> @@ -628,6 +631,15 @@ void kvm_arch_vm_post_create(struct kvm_vm *vm)
>  
>  		vm_sev_ioctl(vm, KVM_SEV_INIT2, &init);
>  	}
> +
> +	if (kvm_has_cap(KVM_CAP_GET_TSC_KHZ)) {

I think we should make this a TEST_REQUIRE(), or maybe even a TEST_ASSERT().
Support for KVM_GET_TSC_KHZ predates KVM selftests by 7+ years.

> +		r = __vm_ioctl(vm, KVM_GET_TSC_KHZ, NULL);
> +		if (r < 0)

Heh, the docs are stale.  KVM hasn't returned an error since commit cc578287e322
("KVM: Infrastructure for software and hardware based TSC rate scaling"), which
again predates selftests by many years (6+ in this case).  To make our lives
much simpler, I think we should assert that KVM_GET_TSC_KHZ succeeds, and maybe
throw in a GUEST_ASSERT(thz_khz) in udelay()?

E.g. as is, if KVM_GET_TSC_KHZ is allowed to fail, then we risk having to deal
with weird failures due to udelay() unexpectedly doing nothing.


> +			tsc_khz = 0;
> +		else
> +			tsc_khz = r;
> +		sync_global_to_guest(vm, tsc_khz);
> +	}
>  }
>  
>  void vcpu_arch_set_entry_point(struct kvm_vcpu *vcpu, void *guest_code)
> -- 
> 2.34.1
>