From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f72.google.com (mail-pj1-f72.google.com [209.85.216.72]) (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 29A5B4457B1 for ; Mon, 10 Aug 2026 22:55:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.72 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786402525; cv=none; b=B9AT0l2Wtbf4qLNPVmCMSk57p4MOgAXO/ueFU0/klXZdHf4aM18zjWT0vsLeLA0X84HTQ/iGAN9EhXsFKz4kydo2Aw0w7LQmVYDqssPLHmgUgGNmSj/Uz43jZnyKH1Z/OSL+F+EjBgPkxyCv5ZQRokImeNlzSN3P9F9emGG2JJc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786402525; c=relaxed/simple; bh=HEa+JxnqvNvLIMvTG8Br8LXOSsVcM74pllbSgSYeaIM=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=T5+H69qL+A7tHBTOAhlHcqQBemXpYM7Y2GvMTIvQPk6vHAjWI56WpvPR9AMnmtOJQWs6kCEnE0i/0CwKlVqFcvfm4L07YjliARN2/wxgBPPiC2TxqJFTOOdi98Q0aCSRlx1DYJL0xVTjgACd6rtqEDC06rEDGt63uRVgK3wUa+A= 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=tQtUMhw7; arc=none smtp.client-ip=209.85.216.72 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="tQtUMhw7" Received: by mail-pj1-f72.google.com with SMTP id 98e67ed59e1d1-38e8fee6af3so2866297a91.1 for ; Mon, 10 Aug 2026 15:55:20 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1786402519; x=1787007319; 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=O2Apit3dTRO/Avi3ufPqsCY55zXXjY9yvVr1aPW3ZCc=; b=tQtUMhw7NGiElkujDBG0t1AWlNBS2ozUJ9QGKq/lvi1TD4ukxVyMWcjg5qfRw4ShfM WPu1Z9k0NF6zP8YjrU/DzOzavVT/H/CJvRQ+3uD+Y3p13zpcaUqT0WscWZzdte98lkqr Z6jNHIJ8mLi7Fig4z//MWd/Y/v0drPBlaROBqR7sdJmOkIyRrXmzvCPA0Bau6KUnHjrr Vj84X624S8vvyO4eN/8WDRiVnvHLfKj0tSlCOaWgcsM0q7Mv1IBAHyadq1kOiefSznQZ +KFhzD6Ip8xysay25sevzxw7pSoKv5drnU8SJDIKaKJjES/Bo6z3SbKXd8gOe92axkRD pmjw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1786402519; x=1787007319; 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=O2Apit3dTRO/Avi3ufPqsCY55zXXjY9yvVr1aPW3ZCc=; b=O6HM1fYyZG+tyLHpy/rXFDMrKuyvoE2GkU9F5LImdkptKZhu1aq7UCEcTs3Ieizum9 r5tSS0qmejodd84zasHMbtQgZZBlOdqIAqzHqU7q78BujPm6wRtE+Xqo/axWLX+3EIHW 7aFNg5k4Zv6MoRW9M5PZy2MTIKbmoKaa6/EhEqhWIiKzn4cDXSknPXMMZ2n48v9Iw/UH nLhSnxvKgc/dBowNUGF0knoBtUU9zYpm2bHXBrH2c4LoQk+vhS3VO6pKy6bpwezh+MBo t5ZeoKQBgPkPl+UKEdVlLnMsepQPRSo50E8HkoJLiSzpq77OCEhh0ogM9jbg2qPPOTsZ sP7g== X-Gm-Message-State: AOJu0YxLqGB70/mHyNthKF6US7/nMuyHjVI10If6YVqTlGnFjHARMH8A jJxRFHwmIDNACvxovRK04x1d4+l1zBuuekqcpRx0P44m6zDiFNuSJ0VxiaqJeTGY9dLDZal9eBN qR3Nwcw== X-Received: from pjbsb12.prod.google.com ([2002:a17:90b:50cc:b0:392:b872:81b9]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:390e:b0:38e:9ef9:eb97 with SMTP id 98e67ed59e1d1-392823ea541mr21595524a91.16.1786402519296; Mon, 10 Aug 2026 15:55:19 -0700 (PDT) Reply-To: Sean Christopherson Date: Mon, 10 Aug 2026 15:54:51 -0700 In-Reply-To: <20260810225500.869288-1-seanjc@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260810225500.869288-1-seanjc@google.com> X-Mailer: git-send-email 2.55.0.679.g6767b8d81c-goog Message-ID: <20260810225500.869288-14-seanjc@google.com> Subject: [PATCH v9 13/21] KVM: x86: Fix compute_guest_tsc() to handle negative time deltas 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 The compute_guest_tsc() function computes the guest TSC at a given kernel_ns timestamp. When the master clock reference point (master_kernel_ns) is earlier than vcpu->arch.this_tsc_nsec, the delta is negative. Since pvclock_scale_delta() takes a u64, the negative value wraps to a huge positive number, producing a wildly wrong result. Handle negative deltas explicitly by scaling the absolute value of the delta and applying it to this_tsc_write with the appropriate sign. This is believed to be unreachable in practice; no path has been identified which invokes compute_guest_tsc() with a timestamp from before the vCPU's TSC generation was established. Fix it for robustness, in the spirit of defence in depth. Signed-off-by: David Woodhouse Signed-off-by: Sean Christopherson --- arch/x86/kvm/x86.c | 14 +++++++++----- 1 file changed, 9 insertions(+), 5 deletions(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 50e92090ba65..76c16a15f059 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1054,11 +1054,15 @@ static int kvm_set_tsc_khz(struct kvm_vcpu *vcpu, u32 user_tsc_khz) static u64 compute_guest_tsc(struct kvm_vcpu *vcpu, s64 kernel_ns) { - u64 tsc = pvclock_scale_delta(kernel_ns-vcpu->arch.this_tsc_nsec, - vcpu->arch.virtual_tsc_mult, - vcpu->arch.virtual_tsc_shift); - tsc += vcpu->arch.this_tsc_write; - return tsc; + s64 delta_ns = kernel_ns - vcpu->arch.this_tsc_nsec; + u64 tsc; + + /* Handle negative deltas gracefully (master clock ref may be earlier) */ + tsc = pvclock_scale_delta(abs(delta_ns), + vcpu->arch.virtual_tsc_mult, + vcpu->arch.virtual_tsc_shift); + + return vcpu->arch.this_tsc_write + (delta_ns >= 0 ? tsc : -tsc); } #ifdef CONFIG_X86_64 -- 2.55.0.679.g6767b8d81c-goog