From: Eduardo Habkost <ehabkost@redhat.com>
To: David Dunn <ddunn@vmware.com>
Cc: "Arjan van de Ven" <arjan@linux.intel.com>,
"KarimAllah Ahmed" <karahmed@amazon.de>,
"Wilson, Matt" <msw@amazon.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"Andi Kleen" <ak@linux.intel.com>,
"Andrea Arcangeli" <aarcange@redhat.com>,
"Andy Lutomirski" <luto@kernel.org>,
"Ashok Raj" <ashok.raj@intel.com>,
"Asit Mallick" <asit.k.mallick@intel.com>,
"Borislav Petkov" <bp@suse.de>,
"Dan Williams" <dan.j.williams@intel.com>,
"Dave Hansen" <dave.hansen@intel.com>,
"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
"H . Peter Anvin" <hpa@zytor.com>,
"Ingo Molnar" <mingo@redhat.com>,
"Janakarajan Natarajan" <Janakarajan.Natarajan@amd.com>,
"Joerg Roedel" <joro@8bytes.org>,
"Jun Nakajima" <jun.nakajima@intel.com>,
"Laura Abbott" <labbott@redhat.com>,
"Linus Torvalds" <torvalds@linux-foundation.org>,
"Masami Hiramatsu" <mhiramat@kernel.org>,
"Paolo Bonzini" <pbonzini@redhat.com>,
"Peter Zijlstra" <peterz@infradead.org>,
"Radim Krčmář" <rkrcmar@redhat.com>,
"Thomas Gleixner" <tglx@linutronix.de>,
"Tim Chen" <tim.c.chen@linux.intel.com>,
"Tom Lendacky" <thomas.lendacky@amd.com>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
"x86@kernel.org" <x86@kernel.org>,
"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
"Fred Jacobs" <fjacobs@vmware.com>,
"Jim Mattson" <jmattson@google.com>,
"David Woodhouse" <dwmw2@infradead.org>
Subject: Re: [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure
Date: Mon, 29 Jan 2018 23:08:32 -0200 [thread overview]
Message-ID: <20180130010832.GA21702@localhost.localdomain> (raw)
In-Reply-To: <7EB9643C-D2DD-477A-90DE-05DC653D2D4B@vmware.com>
On Mon, Jan 29, 2018 at 10:29:28PM +0000, David Dunn wrote:
> On Mon, 2018-01-29 at 13:45:07 -0800, Eduardo Habkost wrote:
>
> > Maybe a generic "family/model/stepping/microcode really matches
> > the CPU you are running on" bit would be useful. The bit could
> > be enabled only on host-passthrough (aka "-cpu host") mode.
> >
> > If we really want to be able to migrate to host with different
> > CPU models (except Skylake), we could add a more specific "we
> > promise the host CPU is never going to be Skylake" bit.
> >
> > Now, if the hypervisor is not providing any of those bits, I
> > would advise against trusting family/model/stepping/microcode
> > under a hypervisor. Using a pre-defined CPU model (that doesn't
> > necessarily match the host) is very common when using KVM VM
> > management stacks.
> >
>
> Eduardo,
>
> I don't see how this is possible in a modern virtualization
> environment.
>
> Under VMware, a VM will be migrated to SkyLake if one is in the
> cluster and supports the features exposed to the VM. This can
> occur for suspend/resume as well.
>
> The migration pool isn't a constant. Hosts can be added to a
> cluster and VMs can be instructed to move across clusters. So
> there doesn't need to be a SkyLake around when the VM powers on
> in order for it to eventually end up on a SkyLake.
If this is the case for your deployment, this means the guest
must never assume it won't run on a Skylake host (even if f/m/s
is not Skylake), doesn't it? Then the hypervisor won't set the
"we promise the host CPU is never going to be Skylake" bit.
>
> Even if we expose bit to indicate that FMS matches the
> underlying host, when does the guest know to query that? The
> VM can be moved at any point in time, including after the guest
> asks if FMS matches host.
If the VM can be moved at any point of time to a different model
of host CPU, this means you won't tell the guest it can trust
f/m/s because it doesn't represent the underlying host. You
won't set the "f/m/s/m really matches the host CPU" bit.
On both scenarios you describe above, it sounds like Linux must
assume it could migrated to a Skylake host at any moment. This
is exactly why I'm proposing those extra bits.
--
Eduardo
next prev parent reply other threads:[~2018-01-30 1:08 UTC|newest]
Thread overview: 75+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-29 22:29 [RFC,05/10] x86/speculation: Add basic IBRS support infrastructure David Dunn
2018-01-29 22:41 ` Andi Kleen
2018-01-29 22:49 ` Jim Mattson
2018-01-30 1:10 ` Eduardo Habkost
2018-01-30 1:20 ` David Dunn
2018-01-30 1:30 ` Eduardo Habkost
2018-01-29 23:51 ` Fred Jacobs
2018-01-30 1:08 ` Eduardo Habkost [this message]
-- strict thread matches above, loose matches on Subject: below --
2018-01-20 19:22 [RFC 00/10] Speculation Control feature support KarimAllah Ahmed
2018-01-20 19:22 ` [RFC 05/10] x86/speculation: Add basic IBRS support infrastructure KarimAllah Ahmed
2018-01-21 14:31 ` Thomas Gleixner
2018-01-21 14:56 ` Borislav Petkov
2018-01-22 9:51 ` Peter Zijlstra
2018-01-22 12:06 ` Borislav Petkov
2018-01-22 13:30 ` Greg Kroah-Hartman
2018-01-22 13:37 ` Woodhouse, David
2018-01-21 15:25 ` David Woodhouse
2018-01-23 20:58 ` David Woodhouse
2018-01-23 22:43 ` Johannes Erdfelt
2018-01-24 8:47 ` Peter Zijlstra
2018-01-24 9:02 ` David Woodhouse
2018-01-24 9:10 ` Greg Kroah-Hartman
2018-01-24 15:09 ` Arjan van de Ven
2018-01-24 15:18 ` David Woodhouse
2018-01-24 9:34 ` Peter Zijlstra
2018-01-24 10:49 ` Henrique de Moraes Holschuh
2018-01-24 12:30 ` David Woodhouse
2018-01-24 12:14 ` David Woodhouse
2018-01-24 12:29 ` Peter Zijlstra
2018-01-24 12:58 ` David Woodhouse
2018-01-29 20:14 ` [RFC,05/10] " Eduardo Habkost
2018-01-29 20:17 ` David Woodhouse
2018-01-29 20:42 ` Eduardo Habkost
2018-01-29 20:44 ` Arjan van de Ven
2018-01-29 21:02 ` David Woodhouse
2018-01-29 21:37 ` Jim Mattson
2018-01-29 21:50 ` Eduardo Habkost
2018-01-29 22:12 ` Jim Mattson
2018-01-30 1:22 ` Eduardo Habkost
2018-01-29 22:25 ` Andi Kleen
2018-01-30 1:37 ` Eduardo Habkost
2018-01-29 21:37 ` Andi Kleen
2018-01-29 21:44 ` Eduardo Habkost
2018-01-29 22:10 ` Konrad Rzeszutek Wilk
2018-01-30 1:12 ` Eduardo Habkost
2018-01-30 0:23 ` Linus Torvalds
2018-01-30 1:03 ` Jim Mattson
2018-01-30 3:13 ` Andi Kleen
2018-01-31 15:03 ` Paolo Bonzini
2018-01-31 15:07 ` Dr. David Alan Gilbert
2018-01-30 1:32 ` Arjan van de Ven
2018-01-30 3:32 ` Linus Torvalds
2018-01-30 12:04 ` Eduardo Habkost
2018-01-30 13:54 ` Arjan van de Ven
2018-01-30 8:22 ` David Woodhouse
2018-01-30 11:35 ` David Woodhouse
2018-01-30 11:56 ` Dr. David Alan Gilbert
2018-01-30 12:11 ` Christian Borntraeger
2018-01-30 14:46 ` Christophe de Dinechin
2018-01-30 14:52 ` Christian Borntraeger
2018-01-30 14:56 ` Christophe de Dinechin
2018-01-30 15:33 ` Christian Borntraeger
2018-01-30 20:46 ` Alan Cox
2018-01-31 10:05 ` Christophe de Dinechin
2018-01-31 10:15 ` Thomas Gleixner
2018-01-31 11:04 ` Dr. David Alan Gilbert
2018-01-31 11:52 ` Borislav Petkov
2018-01-31 12:30 ` Dr. David Alan Gilbert
2018-01-31 13:18 ` Borislav Petkov
2018-01-31 14:04 ` Dr. David Alan Gilbert
2018-01-31 14:44 ` Eduardo Habkost
2018-01-31 16:28 ` Borislav Petkov
2018-01-31 11:07 ` Christophe de Dinechin
2018-01-31 15:00 ` Eduardo Habkost
2018-01-31 15:11 ` Arjan van de Ven
2018-01-31 10:03 ` [RFC 05/10] " Christophe de Dinechin
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