[yocto] [yocto-docs][PATCH] dev-manual: document CPU features and QB_CPU_KVM

[Martin Kelly]

On 04/30/2018 10:25 AM, Scott Rifenbark wrote:
> Hi Martin,
> 
> See 
> https://yoctoproject.org/docs/2.5/dev-manual/dev-manual.html#qemu-kvm-cpu-compatibility 
> and let me know if this section works.  I took your patch and did some 
> rewriting.
> 
> Thanks,
> Scott
> 

I like your changes; thanks!

> On Fri, Apr 27, 2018 at 10:46 AM, Martin Kelly <mkelly at xevo.com 
> <mailto:mkelly at xevo.com>> wrote:
> 
>     As suggested in a recent mail thread by Martin Jansa, there are cases in
>     which QEMU under KVM will crash because of CPU feature incompatibilities
>     between compiled binaries and the host CPU under which qemu is run.
>     Although this is hard to fully escape, we should document the issue to
>     help people work around it.
> 
>     Signed-off-by: Martin Kelly <mkelly at xevo.com <mailto:mkelly at xevo.com>>
>     ---
>       documentation/dev-manual/dev-manual-qemu.xml | 22
>     ++++++++++++++++++++++
>       1 file changed, 22 insertions(+)
> 
>     diff --git a/documentation/dev-manual/dev-manual-qemu.xml
>     b/documentation/dev-manual/dev-manual-qemu.xml
>     index f4719ebc3..1870a6a8d 100644
>     --- a/documentation/dev-manual/dev-manual-qemu.xml
>     +++ b/documentation/dev-manual/dev-manual-qemu.xml
>     @@ -343,6 +343,28 @@
>               </para>
>           </section>
> 
>     +    <section id='qemu-kvm-cpu-compatibility'>
>     +        <title>QEMU CPU Compatiblity under KVM</title>
>     +
>     +        <para>
>     +            By default, the QEMU build compiles for and targets
>     core2duo (for
>     +            64-bit x86) and pentium2 (for 32-bit x86). These CPUS
>     are chose to
>     +            have a broad range of CPU feature compatibility with
>     many commonly
>     +            used CPUs, but in some cases, they still support a CPU
>     feature that
>     +            your host CPU does not. Although this is not a problem
>     when QEMU
>     +            uses software emulation of the feature, it can be when
>     running with
>     +            KVM enabled. Specifically, software compiled with a
>     certain CPU
>     +            feature will crash when run on a CPU under KVM that
>     does not support
>     +            that feature. If this becomes a problem, you can
>     override QEMU's
>     +            runtime CPU setting by changing the
>     <filename>QB_CPU_KVM</filename>
>     +            variable in <filename>qemuboot.conf</filename> in the
>     image deploy
>     +            directory. This setting specifies a
>     <filename>-cpu</filename> option
>     +            passed into QEMU in the <filename>runqemu</filename>
>     script. Running
>     +            <filename>qemu -cpu help</filename> will give a list of
>     supported
>     +            CPU types available.
>     +        </para>
>     +    </section>
>     +
>           <section id='qemu-dev-performance'>
>               <title>QEMU Performance</title>
> 
>     -- 
>     2.11.0
> 
>