From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pg1-f199.google.com (mail-pg1-f199.google.com [209.85.215.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7F4D048987D for ; Wed, 26 Aug 2026 21:33:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.215.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787780003; cv=none; b=NAMNZBDCK9aPXTM3O9UiWeYOamQoskgBX2b8FXeXjcGWPPSGWmFGRetLFUJfYrSFNN9HqCjYGASlZazT+hAgyvwBOEKfoj94IQAjGziKIA8WJBE2EJd1KXyQ5gUdDM2rLDOtNTOJFg90GYmzOrcr/zbT586Pq7dDVjr3IAaD0FI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787780003; c=relaxed/simple; bh=Dyer1ULn/a/i4L4ozp6ZOIcINMbH4yOkx+5FletSldI=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=al0juO5eaErFKuiG5cXL7jiO0VbzgTBD8G7dhCO6i/DWXtFk1DiCXtgEjgNylGM3mrW5CdP1/T/GayT7HWReI5497iUMQVH7Dt2dcASn82Z/eHoADPG2O3XyroGJfg5qAumz6qIOXWgRSu8hWFMfruHDr6NywBGkWgit9A58xs4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=ERe9rvgW; arc=none smtp.client-ip=209.85.215.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="ERe9rvgW" Received: by mail-pg1-f199.google.com with SMTP id 41be03b00d2f7-cc1c2f1acebso52833a12.1 for ; Wed, 26 Aug 2026 14:33:17 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1787779996; x=1788384796; darn=vger.kernel.org; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:reply-to:from:to:cc:subject:date:message-id :reply-to:content-type; bh=goM61cDq5JnxRmzeon6yIX5IBAcd9O/5+qmf+efG8yU=; b=ERe9rvgWNMRNhq8CSzZF0bpo/QSW7+SbU1gXN5J8tzv32LQy5Pj7iJ9fIWGrIf3jc7 uMuC0BByzFxhcuH57U6YvqacPdjbluk7GpxIqgBz2IWqCJHHd9/CdWa60+oa7Q3VnKkA 6TZC9OAF6DBGz27LL31cz1RK9KXB+W3nNtKFUU+oO0XWyl4XxfoSfeTXQIh5svILkzxS Ri0nD4mfmcbwxp0ZE/UvMyLtZ0rOmJKCQLn9HrOS5Ire7BOKAZJbFdIOAkw/2yERTCQc hMC1nzzkVEJ7NdceuymLMK3MUSVIKQS9hR2/BcKopFbzHLagNtgINoXg6/ihZnP6VxZo m1wQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1787779996; x=1788384796; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:reply-to:x-gm-message-state:from:to:cc:subject :date:message-id:reply-to:content-type; bh=goM61cDq5JnxRmzeon6yIX5IBAcd9O/5+qmf+efG8yU=; b=Nxb8bocWA2H1f+3PcDXYKYcMt4fEbiMasHbhnnZJa3kUt/KZCac0s4NR3CEiIAC9Cl D1sY73B4fpbz6MDXoxVjtgEdOMIGDl5Ue8KwBBUzeAWBsAQyDOauQG+UHRL2OnbGqc80 P2n7IChXZMEL8msiD5HCISpEWIywR0kg8sWrBtMfG2jE6/uoKxXPaC961wdeDDgJizRR +XQ3M4N4hhzj4lVcYYLC7fR9ql62uW+pTFw4sLJDkpRsPrT0P4YZo3pH9BxDTPt/1XiP 1nHqrTcIkvqYF1IE5HSgdUthhuEcMVe0e/8K+dRYDYmIZfhVC/UD6ZCgQXdxSmX4yq3u eQOg== X-Gm-Message-State: AFuF++nU63+s06U7WUqwfAUHVjS0nMLPLVWr4dOtNB6xuuB9gemANi2D UY35tUzi8i3Y5k7FXFcn4flVo/3mVRdtnt/o4bOfU9Y59U+IZbguJLbaTcU8jxNUsUgiisTXqj3 KqF++kw== X-Received: from pgjh17.prod.google.com ([2002:a63:df51:0:b0:c8b:2706:11a7]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a20:93a0:b0:3c3:791e:5e18 with SMTP id adf61e73a8af0-3cf83341769mr21745880637.7.1787779996284; Wed, 26 Aug 2026 14:33:16 -0700 (PDT) Reply-To: Sean Christopherson Date: Wed, 26 Aug 2026 14:32:50 -0700 In-Reply-To: <20260826213303.914988-1-seanjc@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260826213303.914988-1-seanjc@google.com> X-Mailer: git-send-email 2.55.0.887.g758fc8c411-goog Message-ID: <20260826213303.914988-11-seanjc@google.com> Subject: [PATCH v10 10/21] KVM: x86: Fall back to non-master-clock if clockread fails in get_kvmclock() From: Sean Christopherson To: Sean Christopherson , Paolo Bonzini Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Paul Durrant , David Woodhouse , Dongli Zhang Content-Type: text/plain; charset="UTF-8" From: David Woodhouse When computing kvmclock and it's currently in master-clock mode, fall back to the non-master-clock path if the clock read fails, e.g. if the kernel's clocksource is transitioning away from TSC but ka->use_master_clock hasn't been updated yet. The rdtsc() fallback was added (well, kept) in commit c68dc1b577ea ("KVM: x86: Report host tsc and realtime values in KVM_GET_CLOCK") purely to avoid uninitialized variables and compilation problems on 32-bit kernels (already addressed). In hindsight, keeping the rdtsc() was a hack and a mistake. Link: https://lore.kernel.org/all/CAOQ_QsgVqS_PuJo8F10Gg5Xw+tKt+5gDx+kJf1j3CiPO4MAOqg@mail.gmail.com Signed-off-by: David Woodhouse [sean: isolate from refactoring changes, write changelog] Signed-off-by: Sean Christopherson --- arch/x86/kvm/x86.c | 13 ++++++------- 1 file changed, 6 insertions(+), 7 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 64df8da3aad4..d8bdc64b31ee 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1676,14 +1676,13 @@ static bool __get_kvmclock_master_clock(struct kvm *kvm, if (!tsc_hz) return false; - if (kvm_get_walltime_and_clockread(&ts, &data->host_tsc)) { - data->realtime = ts.tv_nsec + NSEC_PER_SEC * ts.tv_sec; - data->flags |= KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC; - } else { - data->host_tsc = rdtsc(); - } + if (!kvm_get_walltime_and_clockread(&ts, &data->host_tsc)) + return false; + + data->realtime = ts.tv_nsec + NSEC_PER_SEC * ts.tv_sec; + data->flags |= KVM_CLOCK_REALTIME | KVM_CLOCK_HOST_TSC | + KVM_CLOCK_TSC_STABLE; - data->flags |= KVM_CLOCK_TSC_STABLE; hv_clock.tsc_timestamp = ka->master_cycle_now; hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset; kvm_get_time_scale(NSEC_PER_SEC, tsc_hz, -- 2.55.0.887.g758fc8c411-goog