linux-kernel - Re: kvm: RDTSCP on AMD

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Date:   Mon, 10 Dec 2018 18:08:43 -0200
From:   Eduardo Habkost <ehabkost@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     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>,
        "Daniel P. Berrange" <berrange@...hat.com>,
        Eric Blake <eblake@...hat.com>
Subject: Re: kvm: RDTSCP on AMD

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.

-- 
Eduardo