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[23.128.96.18]) by mx.google.com with ESMTP id v13si8735173edw.250.2021.04.07.15.46.50; Wed, 07 Apr 2021 15:47:14 -0700 (PDT) Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18; Authentication-Results: mx.google.com; dkim=pass header.i=@redhat.com header.s=mimecast20190719 header.b=VJ0NWkNp; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231178AbhDGWj0 (ORCPT + 99 others); Wed, 7 Apr 2021 18:39:26 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:32631 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229449AbhDGWjV (ORCPT ); Wed, 7 Apr 2021 18:39:21 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1617835150; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=RNasGRICqS6NJiBxRNH7M3e6SxbYtc76TWY+YxUdPJQ=; b=VJ0NWkNpXP5AQ8JyWb8nrKZO0wsQ+LqDIp/V6182HA19pOXuWS7es2MqsFQciNuSogmvie CT7YfwUgnqMKIfeCVJSmkCZg4yW7D22DUMhFIkQq1VMlI/ZF/fB9VZT4o9lxkctvKFXwF+ THIAVJZc68fNxzSGaZy2+ReuiUZRtyU= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-294-h_7rBo_JPUGEyMxa_5P5PA-1; Wed, 07 Apr 2021 18:39:08 -0400 X-MC-Unique: h_7rBo_JPUGEyMxa_5P5PA-1 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 03734107ACC7; Wed, 7 Apr 2021 22:39:08 +0000 (UTC) Received: from treble (ovpn-119-205.rdu2.redhat.com [10.10.119.205]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 74C751002388; Wed, 7 Apr 2021 22:39:07 +0000 (UTC) Date: Wed, 7 Apr 2021 17:39:05 -0500 From: Josh Poimboeuf To: Ramakrishna Saripalli Cc: linux-kernel@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH 0/5] Introduce support for PSF mitigation Message-ID: <20210407223905.5dqgzdaibvgq7jeq@treble> References: <20210406155004.230790-1-rsaripal@amd.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20210406155004.230790-1-rsaripal@amd.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 06, 2021 at 10:49:59AM -0500, Ramakrishna Saripalli wrote: > Because PSF speculation is limited to the current program context, > the impact of bad PSF speculation is very similar to that of > Speculative Store Bypass (Spectre v4) > > Predictive Store Forwarding controls: > There are two hardware control bits which influence the PSF feature: > - MSR 48h bit 2 – Speculative Store Bypass (SSBD) > - MSR 48h bit 7 – Predictive Store Forwarding Disable (PSFD) > > The PSF feature is disabled if either of these bits are set. These bits > are controllable on a per-thread basis in an SMT system. By default, both > SSBD and PSFD are 0 meaning that the speculation features are enabled. > > While the SSBD bit disables PSF and speculative store bypass, PSFD only > disables PSF. > > PSFD may be desirable for software which is concerned with the > speculative behavior of PSF but desires a smaller performance impact than > setting SSBD. Hi Ramakrishna, Is there a realistic scenario where an application would want to disable PSF, but not disable SSB? Maybe I'm missing something, but I'd presume an application would either care about this class of attacks, or not. -- Josh