Re: kvm: RDTSCP on AMD

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Dec 11, 2018 at 10:38:39AM +0000, Daniel P. Berrangé wrote:
> 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.

I'd like us to take a step back and consider what exactly
"support" means here.  QEMU will still work on older kernels if
we add RDTSCP to the CPU models, but at the same time I would
like to document that our strict runnability guarantees may not
be kept if running Linux < 4.5.

-- 
Eduardo