linux-kernel - Re: kvm: RDTSCP on AMD

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Message-ID: <20181211143523.GH921@redhat.com>
Date:   Tue, 11 Dec 2018 14:35:23 +0000
From:   Daniel P. Berrangé <berrange@...hat.com>
To:     Eduardo Habkost <ehabkost@...hat.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jörg Rödel <joro@...tes.org>,
        Andre Przywara <andre.przywara@....com>,
        kvm ML <kvm@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Eric Blake <eblake@...hat.com>
Subject: Re: kvm: RDTSCP on AMD

On Tue, Dec 11, 2018 at 09:55:46AM -0200, Eduardo Habkost wrote:
> 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
> > > >  
> > > > +@...e System requirements
> > > > +@...pter System requirements
> > > > +
> > > > +@...tion 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.

I've just noticed that this thread is not in fact on qemu-devel. So
rather than debating the meaning of support here, lets discuss it
once the patch adding rdtscp to OpteronG* CPU model is posted to
qemu-devel.

Regards,
Daniel
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