Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future
 CPUs
To:     "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>,
        "valdis.kletnieks@vt.edu" <valdis.kletnieks@vt.edu>
Cc:     Jon Masters <jcm@jonmasters.org>,
        David Woodhouse <dwmw2@infradead.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "x86@kernel.org" <x86@kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        Ingo Molnar <mingo@kernel.org>
References: <1518449255-2182-1-git-send-email-dwmw@amazon.co.uk>
 <1518509708.12890.33.camel@infradead.org>
 <27c85759-e662-d281-f8a0-0a80ca8ee18f@redhat.com>
 <1518517262.12890.43.camel@infradead.org>
 <1518518198.12890.48.camel@infradead.org>
 <02bd3fdd-1b73-6cab-fb09-38ba933396bd@redhat.com>
 <1518775136.7876.13.camel@infradead.org>
 <75287047-77a0-e0ff-c2e8-61c81641251e@redhat.com>
 <1518776517.7876.21.camel@infradead.org>
 <d61edc94-8a51-62bd-65ed-8068451ca5b5@redhat.com>
 <1518783021.7876.34.camel@infradead.org>
 <4882860e-23c5-75b3-ac02-c700f615156e@jonmasters.org>
 <0575AF4FD06DD142AD198903C74E1CC87A61923C@ORSMSX103.amr.corp.intel.com>
 <10373.1519084385@turing-police.cc.vt.edu>
 <0575AF4FD06DD142AD198903C74E1CC87A619255@ORSMSX103.amr.corp.intel.com>
 <aa3fbaf1-0efc-64b1-4f7b-3d00b0493d48@redhat.com>
 <0575AF4FD06DD142AD198903C74E1CC87A619465@ORSMSX103.amr.corp.intel.com>
 <2159cdc0-c30d-3bf8-1c25-74bff46a1e91@redhat.com>
 <0575AF4FD06DD142AD198903C74E1CC87A6194AB@ORSMSX103.amr.corp.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <5b39cfbb-a04f-46a8-87cb-3a52fc8e9700@redhat.com>
Date:   Tue, 20 Feb 2018 16:09:38 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.6.0
MIME-Version: 1.0
In-Reply-To: <0575AF4FD06DD142AD198903C74E1CC87A6194AB@ORSMSX103.amr.corp.intel.com>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On 20/02/2018 15:59, Van De Ven, Arjan wrote:
> 
>>> I meant software wise. You're not going to live migrate from xen to
>>> kvm or backwards. or between very radically different versions of the
>>> kvm stack.
>>
>> Forwards migration to a radically newer version certainly happens.  So
>> when the source hypervisor was too old to tell the VM about IBRS_ALL,
>> for example, migration should work properly and the VM should perform
>> well on the destination hypervisor.
> 
> that makes sense, compatibility in this direction can be done
> and is useful as you move a fleet of servers forward
> 
>> Backwards migration to older hypervisors also happens sometimes, but in
>> general it creates more userspace than kernel issues.
> 
> that direction is obviously harder

From KVM's point of view it's mostly a matter of having a complete ioctl
API right.  I'm not that much worried by it.

Paolo