Re: kvm: RDTSCP on AMD

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Eduardo Habkost <ehabkost@redhat.com>
Cc: "Borislav Petkov" <bp@alien8.de>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Jörg Rödel" <joro@8bytes.org>,
	"Andre Przywara" <andre.przywara@arm.com>,
	"kvm ML" <kvm@vger.kernel.org>,
	lkml <linux-kernel@vger.kernel.org>,
	"Tom Lendacky" <thomas.lendacky@amd.com>,
	"Eric Blake" <eblake@redhat.com>
Subject: Re: kvm: RDTSCP on AMD
Date: Tue, 11 Dec 2018 10:38:39 +0000	[thread overview]
Message-ID: <20181211103839.GC921@redhat.com> (raw)
In-Reply-To: <20181210200843.GN4669@habkost.net>

On Mon, Dec 10, 2018 at 06:08:43PM -0200, Eduardo Habkost wrote:
> On Mon, Dec 10, 2018 at 08:42:58PM +0100, Borislav Petkov wrote:
> > On Mon, Dec 10, 2018 at 05:06:00PM -0200, Eduardo Habkost wrote:
> > > I mean documenting it.  We already have code that will print
> > > warnings if a feature isn't available.
> > > 
> > > See my previous attempt to document the minimum kernel version
> > > at <https://www.mail-archive.com/qemu-devel@nongnu.org/msg486559.html>.
> > 
> > Something like this?
> > 
> > I still have no clue how to add RDTSCP on pc-*-4.0 and newer.
> 
> qemu.git master have no PC_COMPAT_3_1 and pc-*-4.0 machine-types
> yet.  I've sent another reply with additional pointers.
> 
> > 
> > Thx.
> > 
> > ---
> > diff --git a/qemu-doc.texi b/qemu-doc.texi
> > index f7ad1dfe4b69..913b2ef68d54 100644
> > --- a/qemu-doc.texi
> > +++ b/qemu-doc.texi
> > @@ -37,6 +37,7 @@
> >  * QEMU System emulator for non PC targets::
> >  * QEMU Guest Agent::
> >  * QEMU User space emulator::
> > +* System requirements::
> >  * Implementation notes::
> >  * Deprecated features::
> >  * Supported build platforms::
> > @@ -2813,6 +2814,13 @@ Act as if the host page size was 'pagesize' bytes
> >  Run the emulation in single step mode.
> >  @end table
> >  
> > +@node System requirements
> > +@chapter System requirements
> > +
> > +@section KVM kernel module
> > +
> > +On x86_64 hosts, the default set of CPU features enabled by the KVM accelerator
> > +require the host to be running Linux v4.5 or newer.
> >  
> 
> Sounds good, but it would be nice to document what exactly makes
> Linux 4.5+ necessary.  e.g.:
> 
> | On x86_64 hosts, it is recommended to run Linux 4.5 or newer on
> | the host when using the KVM accelerator.
> | 
> | The Opteron_G2, OpteronG3, Opteron_G4, and Opteron_G5 CPU
> | models require KVM support for RDTSCP, which was added on
> | Linux 4.5.
> 
> I'm CCing some people who may help review English grammar and
> style.

Note that QEMU has a declared supported platform list which is used to
set the minimum software versions required.

   https://qemu.weilnetz.de/doc/qemu-doc.html#Linux-OS

Based on that policy, the Linux distros we aim to support have the
following kernel versions:

  - Debian Jessie: 4.5
  - Debian Stretch: 4.5
  - RHEL-7: 3.10
  - Ubuntu LTS (Xenial): 4.5
  - OpenSUSE Leap 15: 4.12.14

So we can't require Linux 4.5 as a minimum version at this time.

Regards,
Daniel
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