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[91.219.240.17]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-82f8e9d35acsm17671348b3a.15.2026.04.22.05.36.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 22 Apr 2026 05:36:29 -0700 (PDT) From: Vitaly Kuznetsov To: Piotr Zarycki , kvm@vger.kernel.org Cc: seanjc@google.com, pbonzini@redhat.com, shuah@kernel.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org, Piotr Zarycki Subject: Re: [PATCH] KVM: selftests: hyperv_tlb_flush: replace NOP loop with udelay() In-Reply-To: <20260421172953.191345-1-piotr.zarycki@gmail.com> References: <20260421172953.191345-1-piotr.zarycki@gmail.com> Date: Wed, 22 Apr 2026 14:36:17 +0200 Message-ID: <87v7djgoji.fsf@redhat.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Piotr Zarycki 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 > --- > 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 -- Vitaly