[PATCH] KVM: selftests: hyperv_tlb_flush: replace NOP loop with udelay()

[PATCH] KVM: selftests: hyperv_tlb_flush: replace NOP loop with udelay()

[Piotr Zarycki]

Replace the open-coded NOP loop with udelay() which was added to KVM
selftests in commit 6b878cbb87bf ("KVM: selftests: Add guest udelay()
utility for x86"). The NOP loop is CPU speed dependent while udelay()
provides a deterministic delay regardless of host CPU frequency.

Signed-off-by: Piotr Zarycki <piotr.zarycki@gmail.com>
---
 tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c b/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c
index c542cc4762b1..68ebd790ff41 100644
--- a/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c
+++ b/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c
@@ -141,15 +141,9 @@ static void swap_two_test_pages(vm_paddr_t pte_gva1, vm_paddr_t pte_gva2)
 	*(uint64_t *)pte_gva2 = tmp;
 }
 
-/*
- * TODO: replace the silly NOP loop with a proper udelay() implementation.
- */
 static inline void do_delay(void)
 {
-	int i;
-
-	for (i = 0; i < 1000000; i++)
-		asm volatile("nop");
+	udelay(100);
 }
 
 /*
-- 
2.53.0

Re: [PATCH] KVM: selftests: hyperv_tlb_flush: replace NOP loop with udelay()

[Dan Carpenter]

On Tue, Apr 21, 2026 at 07:29:53PM +0200, Piotr Zarycki wrote:
> Replace the open-coded NOP loop with udelay() which was added to KVM
> selftests in commit 6b878cbb87bf ("KVM: selftests: Add guest udelay()
> utility for x86"). The NOP loop is CPU speed dependent while udelay()
> provides a deterministic delay regardless of host CPU frequency.
> 
> Signed-off-by: Piotr Zarycki <piotr.zarycki@gmail.com>
> ---

How did you test this?

regards,
dan carpenter

Re: [PATCH] KVM: selftests: hyperv_tlb_flush: replace NOP loop with udelay()

[Piotr Zarycki]

On Tue, Apr 21, 2026 at 11:10:22PM +0300, Dan Carpenter wrote:
> 
> How did you test this?
> 


  I built the test using:

    make -C tools/testing/selftests/kvm ARCH=x86_64 x86/hyperv_tlb_flush

  And ran it 100 times to verify it passes consistently

    results: 100/100 passed, 0 failed
    average time: 466ms

  For comparison, the original NOP loop averaged ~537ms on the same machine.

Re: [PATCH] KVM: selftests: hyperv_tlb_flush: replace NOP loop with udelay()

[Vitaly Kuznetsov]

Piotr Zarycki <piotr.zarycki@gmail.com> writes:

> Replace the open-coded NOP loop with udelay() which was added to KVM
> selftests in commit 6b878cbb87bf ("KVM: selftests: Add guest udelay()
> utility for x86"). The NOP loop is CPU speed dependent while udelay()
> provides a deterministic delay regardless of host CPU frequency.
>
> Signed-off-by: Piotr Zarycki <piotr.zarycki@gmail.com>
> ---
>  tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c b/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c
> index c542cc4762b1..68ebd790ff41 100644
> --- a/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c
> +++ b/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c
> @@ -141,15 +141,9 @@ static void swap_two_test_pages(vm_paddr_t pte_gva1, vm_paddr_t pte_gva2)
>  	*(uint64_t *)pte_gva2 = tmp;
>  }
>  
> -/*
> - * TODO: replace the silly NOP loop with a proper udelay() implementation.
> - */
>  static inline void do_delay(void)
>  {
> -	int i;
> -
> -	for (i = 0; i < 1000000; i++)
> -		asm volatile("nop");
> +	udelay(100);
>  }
>  
>  /*

Oh, I completely forgot I added the TODO :-) Thanks for the patch! I
also gave it a spin and it looks like rdtsc() from udelay() doesn't
hurt.

Personally, I belive we should now just drop the whole do_delay()
function and use udelay(100) instead. We have only two call sites and
there's no strict requirement that the delay time should match. 

In any case,

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

-- 
Vitaly

[PATCH v2] KVM: selftests: hyperv_tlb_flush: replace NOP loop with udelay()

[Piotr Zarycki]

Replace the open-coded NOP loop with udelay() which was added to KVM
selftests in commit 6b878cbb87bf ("KVM: selftests: Add guest udelay()
utility for x86"). The NOP loop is CPU speed dependent while udelay()
provides a deterministic delay regardless of host CPU frequency.

Signed-off-by: Piotr Zarycki <piotr.zarycki@gmail.com>
---
 .../testing/selftests/kvm/x86/hyperv_tlb_flush.c  | 15 ++-------------
 1 file changed, 2 insertions(+), 13 deletions(-)

diff --git a/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c b/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c
index c542cc4762b1..4b7f089542c1 100644
--- a/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c
+++ b/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c
@@ -141,17 +141,6 @@ static void swap_two_test_pages(vm_paddr_t pte_gva1, vm_paddr_t pte_gva2)
 	*(uint64_t *)pte_gva2 = tmp;
 }
 
-/*
- * TODO: replace the silly NOP loop with a proper udelay() implementation.
- */
-static inline void do_delay(void)
-{
-	int i;
-
-	for (i = 0; i < 1000000; i++)
-		asm volatile("nop");
-}
-
 /*
  * Prepare to test: 'disable' workers by setting the expectation to '0',
  * clear hypercall input page and then swap two test pages.
@@ -169,7 +158,7 @@ static inline void prepare_to_test(struct test_data *data)
 	wmb();
 
 	/* Make sure workers have enough time to notice */
-	do_delay();
+	udelay(100);
 
 	/* Swap test page mappings */
 	swap_two_test_pages(data->test_pages_pte[0], data->test_pages_pte[1]);
@@ -189,7 +178,7 @@ static inline void post_test(struct test_data *data, u64 exp1, u64 exp2)
 	set_expected_val((void *)data->test_pages, exp2, WORKER_VCPU_ID_2);
 
 	/* Make sure workers have enough time to test */
-	do_delay();
+	udelay(100);
 }
 
 #define TESTVAL1 0x0101010101010101
-- 
2.53.0

Re: [PATCH v2] KVM: selftests: hyperv_tlb_flush: replace NOP loop with udelay()

[Vitaly Kuznetsov]

Piotr Zarycki <piotr.zarycki@gmail.com> writes:

> Replace the open-coded NOP loop with udelay() which was added to KVM
> selftests in commit 6b878cbb87bf ("KVM: selftests: Add guest udelay()
> utility for x86"). The NOP loop is CPU speed dependent while udelay()
> provides a deterministic delay regardless of host CPU frequency.
>
> Signed-off-by: Piotr Zarycki <piotr.zarycki@gmail.com>

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>

Thanks!

> ---
>  .../testing/selftests/kvm/x86/hyperv_tlb_flush.c  | 15 ++-------------
>  1 file changed, 2 insertions(+), 13 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c b/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c
> index c542cc4762b1..4b7f089542c1 100644
> --- a/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c
> +++ b/tools/testing/selftests/kvm/x86/hyperv_tlb_flush.c
> @@ -141,17 +141,6 @@ static void swap_two_test_pages(vm_paddr_t pte_gva1, vm_paddr_t pte_gva2)
>  	*(uint64_t *)pte_gva2 = tmp;
>  }
>  
> -/*
> - * TODO: replace the silly NOP loop with a proper udelay() implementation.
> - */
> -static inline void do_delay(void)
> -{
> -	int i;
> -
> -	for (i = 0; i < 1000000; i++)
> -		asm volatile("nop");
> -}
> -
>  /*
>   * Prepare to test: 'disable' workers by setting the expectation to '0',
>   * clear hypercall input page and then swap two test pages.
> @@ -169,7 +158,7 @@ static inline void prepare_to_test(struct test_data *data)
>  	wmb();
>  
>  	/* Make sure workers have enough time to notice */
> -	do_delay();
> +	udelay(100);
>  
>  	/* Swap test page mappings */
>  	swap_two_test_pages(data->test_pages_pte[0], data->test_pages_pte[1]);
> @@ -189,7 +178,7 @@ static inline void post_test(struct test_data *data, u64 exp1, u64 exp2)
>  	set_expected_val((void *)data->test_pages, exp2, WORKER_VCPU_ID_2);
>  
>  	/* Make sure workers have enough time to test */
> -	do_delay();
> +	udelay(100);
>  }
>  
>  #define TESTVAL1 0x0101010101010101

-- 
Vitaly