From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Google-Smtp-Source: AH8x224dwMRgmv+u/ALjmcnd0zbxCsCkwLBhAtY05ELmfCzEyED8GS5kQsYdE1S2Lvnw0+ag5IZJ ARC-Seal: i=1; a=rsa-sha256; t=1517435959; cv=none; d=google.com; s=arc-20160816; b=McKNz1z5pgCw7+sE5SM46S6xaBHikLviVtuLOUkYDI2/WfjzM50Pow3owFz6K+suT4 30GjHLX3W5clBuP6f6Wpp+EbFb2GRzR+maV6byFSOxJlGW63VOh5zyTvWCg9tIzDGfDH oziLDymYpr1Ay7GuWjbsWMXYvXVGx8326U9PCA+yrt1RQAwLczLN8HgISNge1qYoYgE2 NjzSkRjB/2Chgte09GR+IvrPmkQiAo6HmcPMxXAwAbEeW7lyXvgwrccf8OtkIpqxxaz1 hfMQsil9M2RTf5BHg/1ypGavRRJIW8yhFD89iKtOqoNpHvHdVrKr5Ql4R3QL3kiWHBr6 3Uuw== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=content-transfer-encoding:content-language:in-reply-to:mime-version :user-agent:date:message-id:from:references:cc:to:subject :arc-authentication-results; bh=mfoqFvfelUrSCJJSpRlSznYowB0lbMRYPuys/kmlwzY=; b=rV/7ImGjvDc4p0JgXWGOokDV4aYbtWYzgdcSKmydI4rDKbIhEKVZVXs/GlQnEyJukb SieU4VgGRdWwfLio+jJVQ5zyXDxrgJxI/81KlNP/4/zAZuNjOzD5jdsUXtfJ7dLEVvIR MsxImvkXYJFViCrWU656R5Mpo7KYYy3qOrBu5z19bNPGbD24PFlI8kLmWGxF4KQydzKf FNtOYVwUeZ+/KJX+0GFrlLkcvVw0oIUwWR4FpbJNFF4lb3IsOQwtmk09V3JZ8QNhzAE+ D86GXDKHCGPQfNCuTXZGFpau5If08YQzDNrhrLfIuBvCk2CRukfW50wN5qMznp62TKmz DgXA== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: domain of pbonzini@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=pbonzini@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Authentication-Results: mx.google.com; spf=pass (google.com: domain of pbonzini@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=pbonzini@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Subject: Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL To: Jim Mattson Cc: KarimAllah Ahmed , KarimAllah Ahmed , kvm list , LKML , the arch/x86 maintainers , Asit Mallick , Arjan Van De Ven , Dave Hansen , Andi Kleen , Andrea Arcangeli , Linus Torvalds , Tim Chen , Thomas Gleixner , Dan Williams , Jun Nakajima , David Woodhouse , Greg KH , Andy Lutomirski , Ashok Raj References: <1517427467-28567-1-git-send-email-karahmed@amazon.de> <1517427467-28567-5-git-send-email-karahmed@amazon.de> <06cb88da-f355-41ed-380f-7daa8ddf6159@amazon.com> <67dca2bb-1298-c7fa-f681-018e4edc6725@redhat.com> <16d89e09-fbfc-4a08-00a6-40cfb4776018@redhat.com> From: Paolo Bonzini Message-ID: Date: Wed, 31 Jan 2018 16:59:10 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: =?utf-8?q?1591113692084885209?= X-GMAIL-MSGID: =?utf-8?q?1591146928512779600?= X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On 31/01/2018 16:53, Jim Mattson wrote: > On Wed, Jan 31, 2018 at 1:42 PM, Paolo Bonzini wrote: > >> Can we just say it sucks to be L2 too? :) Because in the end as long as >> no one ever writes to spec_ctrl, everybody is happy. > > Unfortunately, quite a few OS vendors shipped IBRS-based mitigations > earlier this month. (Has Redhat stopped writing to IA32_SPEC_CTRL yet? > :-) Not yet, but getting there. :) > And in the long run, everyone is going to set IA32_SPEC_CTRL.IBRS=1 on > CPUs with IA32_ARCH_CAPABILITIES.IBRS_ALL. And then it will suck for everyone---they will have to pay the price of saving/restoring an MSR that is going to be written just once. Perhaps we will have to tweak the heuristic, only passing IBRS through when the guest writes IBRS=0. In the end I think it's premature to try and optimize for L2 guests of long-lived L1 hypervisors. Paolo