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    SubjectRe: kvm: RDTSCP on AMD
    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

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