From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pf1-f201.google.com (mail-pf1-f201.google.com [209.85.210.201]) (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 0EAC73B635A for ; Wed, 1 Jul 2026 20:13:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782936793; cv=none; b=NBB94FmwZPPciEQEAUESNrJzgTPT5a9de2AR0suUWFKwJqBeSdhA8Hc8M3RqVsBYq16fMsOQDhhbh214O7fJ30jAATf/0TBNgDn8yFX9Cb6UYAgpgA2HVIjeoM7Dh93BdsK0Hhq731Rs1/HeFgo12eb2z2M8s0GRabCiQUumJ+c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782936793; c=relaxed/simple; bh=cF/jjZEyvGUvgS6uTcZoamdwydDhtZu3BqFwF+APe0M=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=n2tSAQgnFSk5QLx1TFTlX2OstAOc7DRDnrj23LD01ZRu16yGJ/CBexHwmzp5jceRtSj9e3o0tJpkSslqcvi59VUTIIqiIiWBTDOql9oA9cjwx2CCpDkxQiBlF9tp02F+xRPUkhTspL66AWq7Y9NcGMsOdVKFyWRrBt+1HxZJEPg= 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=ZSrHxd7g; arc=none smtp.client-ip=209.85.210.201 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="ZSrHxd7g" Received: by mail-pf1-f201.google.com with SMTP id d2e1a72fcca58-8423f544944so814089b3a.3 for ; Wed, 01 Jul 2026 13:13:11 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1782936791; x=1783541591; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=Gd2T2yrKrdjkGkPBFuv19B1+1Czc0J5Qs+z0vXV9YM8=; b=ZSrHxd7gpGFYvcMF8vIkZgw8gbToxaYXQ5Ff9P9lfqWRbySSYlLpJW/PGDWkjCHAJW ogthRIVIu6zZj7SuRpWRlITSrgm13ZqOxuonKtFH4z0J9jfw6fSm/eJVNA6R8LrjN8Lb uwVQhpwmL65wkRwdgaO/nQD5cAZNllYo7zq0t8AHTEwlrNAnwVYmHc5TICqpDQU4kbSU zVaIW9q+whT51m6JOGl+MvG/DHBLMSb2XcPGyMgclC+I3QX2tufMEWwY1tXWubcPnGvq h6Z0a8F7QdrsOf58T5cbqDM+Ng2teK3JXU7JBoI+Ur5oPUkRFy5OZ7GuIIqciGmcaMlZ oj+g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1782936791; x=1783541591; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=Gd2T2yrKrdjkGkPBFuv19B1+1Czc0J5Qs+z0vXV9YM8=; b=G5eYODbQu3S/QRDwCBd6Z9ZsuEYMiQ7WZexQnA4KVcwP/Tc2SHsuHMaFE5pf0Q3f8t v9Nsnh5Hm+XG8HUXf1clC9iTvOk/qBFO8aREtod3t8UoTNYUqL2hY4DfU2/P7KQFaxI+ Xg42Kupsg5R6YYvL88+D94n5BkgOiN3honM3qlHs/w9cN5SlrmDxpZPboq/QQUqXqI5s ANLewifS3hznFUiDQIGrfL0SeWxNddTcJNR/icP13Ilq7oOblx/V3DHnpjicn2dutUxP ulMC1QV2zSNVMGhygoiOhNH8tQUQQmQIYeKErqQqRgHujbXB8V+UZjkiWsSXSjkzIYkC 8F8w== X-Forwarded-Encrypted: i=1; AHgh+RqhTiRTZf606cE4wQhros2BduFjYQ8UuaqdICVkj34C7IVWO6d0vlkDzn1g0cbwyEhUqF0=@vger.kernel.org X-Gm-Message-State: AOJu0YzYHXWuMN3lBBHLS0fR4LuWdym/mE1gJKr9fQfrR7yLcZXwSsDe tUUL3vnNAirbuhAyJWNMfqy3e4LRmyWCOi97LMMi0Ii0gGJY89wuO/GbAPV2ejZUf7JOuHxJQX4 ObR3uOQ== X-Received: from pfbil2.prod.google.com ([2002:a05:6a00:8d42:b0:847:aa6c:47fc]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a00:2d06:b0:845:44ec:a648 with SMTP id d2e1a72fcca58-847c506fffdmr2026726b3a.21.1782936791019; Wed, 01 Jul 2026 13:13:11 -0700 (PDT) Date: Wed, 1 Jul 2026 13:13:10 -0700 In-Reply-To: <20260701200321.256EF1F000E9@smtp.kernel.org> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260701193212.749551-1-seanjc@google.com> <20260701193212.749551-23-seanjc@google.com> <20260701200321.256EF1F000E9@smtp.kernel.org> Message-ID: Subject: Re: [PATCH v5 22/51] x86/kvm: Mark TSC as reliable when it's constant and nonstop From: Sean Christopherson To: sashiko-reviews@lists.linux.dev Cc: linux-hyperv@vger.kernel.org, kvm@vger.kernel.org Content-Type: text/plain; charset="us-ascii" On Wed, Jul 01, 2026, sashiko-bot@kernel.org wrote: > > diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c > > index 909d3e5e5bcd5..1cef54e1e7d9e 100644 > > --- a/arch/x86/kernel/kvm.c > > +++ b/arch/x86/kernel/kvm.c > [ ... ] > > @@ -1040,7 +1041,16 @@ static void __init kvm_init_platform(void) > > } > > } > > > > - kvmclock_init(); > > + /* > > + * If the TSC counts at a constant frequency across P/T states and in > > + * deep C-states, treat the TSC reliable, as guaranteed by KVM. > > + */ > > + tsc_is_reliable = boot_cpu_has(X86_FEATURE_CONSTANT_TSC) && > > + boot_cpu_has(X86_FEATURE_NONSTOP_TSC); > > + if (tsc_is_reliable) > > + setup_force_cpu_cap(X86_FEATURE_TSC_RELIABLE); > > [Severity: High] > Does setting X86_FEATURE_TSC_RELIABLE here bypass SMP cross-CPU TSC > synchronization checks? No, KVM_FEATURE_CLOCKSOURCE_STABLE_BIT is for kvmclock, not for TSC. The CONSTANT+NONSTOP combination is KVM's way of guaranteeing cross-CPU synchronization. > Forcing X86_FEATURE_TSC_RELIABLE based only on CONSTANT_TSC and NONSTOP_TSC, > without verifying if the hypervisor actually guarantees cross-CPU > synchronization (e.g., via KVM_FEATURE_CLOCKSOURCE_STABLE_BIT), sets > tsc_clocksource_reliable = 1. > > This bypasses the synchronization check in check_tsc_sync_source(): > > arch/x86/kernel/tsc_sync.c:check_tsc_sync_source() { > ... > if (tsc_store_and_check_tsc_adjust(false) || tsc_clocksource_reliable) > return; > ... > } > > Could this cause time to go backwards for guests running on hosts with > unsynchronized TSCs when threads migrate between vCPUs?