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From: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
To: "Van De Ven, Arjan" <arjan.van.de.ven@intel.com>
Cc: "valdis.kletnieks@vt.edu" <valdis.kletnieks@vt.edu>,
	Jon Masters <jcm@jonmasters.org>,
	David Woodhouse <dwmw2@infradead.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	"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>
Subject: Re: [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs
Date: Tue, 20 Feb 2018 00:13:01 +0000	[thread overview]
Message-ID: <20180220001301.70101e1d@alans-desktop> (raw)
In-Reply-To: <0575AF4FD06DD142AD198903C74E1CC87A619255@ORSMSX103.amr.corp.intel.com>

On Tue, 20 Feb 2018 00:00:23 +0000
"Van De Ven, Arjan" <arjan.van.de.ven@intel.com> wrote:

> > On Mon, 19 Feb 2018 23:42:24 +0000, "Van De Ven, Arjan" said:
> >   
> > > the guest is not the problem; guests obviously will already honor if Enhanced
> > > IBRS is enumerated. The problem is mixed migration pools where the  
> > hypervisor  
> > > may need to decide to not pass this enumeration through to the guest.  
> > 
> > For bonus points:  What should happen to a VM that is live migrated from one
> > hypervisor to another, and the hypervisors have different IBRS support?  
> 
> Doctor Doctor it hurts when I do this....
> 
> Migration tends to only work between HV's that are relatively homogeneous, that's nothing new... folks who run clouds or bigger pools know this obviously.

In theory there's nothing stopping a guest getting a 'you are about to
gain/lose IBRS' message or having a new 'CPU' hotplugged and the old one
removed.

It's just that it's always been less painful to avoid the problems you
get with wild mismatches than attmept the cure. And any 'cure' goes
beyond the OS kernel into some of the managed runtimes so I'd agree with
Arjan 'don't do that'.

Alan

  reply	other threads:[~2018-02-20  0:13 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-12 15:27 [PATCH 1/2] x86/speculation: Correct Speculation Control microcode blacklist again David Woodhouse
2018-02-12 15:27 ` [PATCH 2/2] x86/speculation: Support "Enhanced IBRS" on future CPUs David Woodhouse
2018-02-13  7:47   ` Ingo Molnar
2018-02-13  8:12     ` David Woodhouse
2018-02-13  8:02   ` Paolo Bonzini
2018-02-13  8:15     ` David Woodhouse
2018-02-13  9:58       ` Paolo Bonzini
2018-02-13 10:21         ` David Woodhouse
2018-02-13 10:36           ` David Woodhouse
2018-02-13 10:41             ` Paolo Bonzini
2018-02-13 10:53               ` David Woodhouse
2018-02-13 10:55                 ` Paolo Bonzini
2018-02-16  9:58               ` David Woodhouse
2018-02-16 10:08                 ` Paolo Bonzini
2018-02-16 10:21                   ` David Woodhouse
2018-02-16 11:04                     ` Paolo Bonzini
2018-02-16 12:10                       ` David Woodhouse
2018-02-19 23:37                         ` Jon Masters
2018-02-19 23:42                           ` Van De Ven, Arjan
2018-02-19 23:53                             ` valdis.kletnieks
2018-02-20  0:00                               ` Van De Ven, Arjan
2018-02-20  0:13                                 ` Alan Cox [this message]
2018-02-20  0:43                                   ` Linus Torvalds
2018-02-20  1:03                                     ` Alan Cox
2018-02-20  1:08                                       ` Van De Ven, Arjan
2018-02-20  8:52                                     ` Thomas Gleixner
2018-02-20 11:43                                 ` Paolo Bonzini
2018-02-20 14:08                                   ` Van De Ven, Arjan
2018-02-20 14:46                                     ` Paolo Bonzini
2018-02-20 14:59                                       ` Van De Ven, Arjan
2018-02-20 15:09                                         ` Paolo Bonzini
2018-02-23 18:12                                       ` Is: RSB Alternative bit in IA32_ARCH_CAPABILITIES Was:Re: " Konrad Rzeszutek Wilk
2018-02-23 18:18                                         ` Van De Ven, Arjan
2018-02-15 15:21     ` Pavel Machek
2018-02-13  8:57 ` [tip:x86/pti] x86/speculation: Correct Speculation Control microcode blacklist again tip-bot for David Woodhouse

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