Re: [PATCH v7] xen/sm{e, a}p: allow disabling sm{e, a}p for Xen itself

[He Chen <he.chen@linux.intel.com>]

On Mon, Oct 10, 2016 at 06:16:41AM -0600, Jan Beulich wrote:
> >>> On 09.10.16 at 10:20, <he.chen@linux.intel.com> wrote:
> > Changes in v7:
> > * bugfix: fix the bug that this patch doesn't work on machine without SMAP.
> > * test: This patch has not been tested (on 32-bit PV environment).
> >         Really sorry for that since I have took several days trying to
> >         setup a 32-bit PV guest but finally failed.
> 
> Well, I don't know what to say. And since you don't say what your
> problem is/was, I also don't see how anyone could help.
> 
Apologies for sending out a patch without testing it entirely.
To be honest, I am not so fimilar with 32-bit PV guest...
I create a HVM guest and then compile linux kernel with the configuration
suggested by https://wiki.xen.org/wiki/Mainline_Linux_Kernel_Configs
after that, I copy vmlinuz and initramfs from guest to host and I boot
guest with PV cfg file below:
```
name="rhel32"
memory=2048
vcpus=2
on_crash="destroy"
on_poweroff="destroy"
on_reboot="restart"
localtime=0
builder="linux"
kernel="/root/xen_guest/rhel32pv/vmlinuz-4.8.0+"
ramdisk="/root/xen_guest/rhel32pv/initramfs-4.8.0+.img"
extra="root=/dev/xvda"
disk=['file:/root/xen_guest/rhel32pv/rhel32.img,xvda,w',]
vif=[ 'mac=00:15:3e:22:f5:1b','bridge=xenbr0']
```
The guest fail to boot and error message says can not find modules in
/lib/module...

I also try to install a PV guest directly by netboot way, but I am
blocked at "installing kernel" step, error shows and let me try.

The problems are trivial, and may I ask for some "how to build a 32-bit
PV guest on 64-bit host" reference wiki page or threads?

> > @@ -1404,12 +1448,16 @@ void __init noreturn __start_xen(unsigned long mbi_p)
> >  
> >      if ( !opt_smep )
> >          setup_clear_cpu_cap(X86_FEATURE_SMEP);
> > -    if ( cpu_has_smep )
> > +    else if ( cpu_has_smep && opt_smep == 1 )
> 
> How about
> 
>     if ( cpu_has_smep && opt_smep != SMEP_HVM_ONLY )
> 
> (i.e. I dislike both the "else" and the hard-coded 1)? Or if you dislike
> this, then at least > 0 instead of == 1 please, or provide a #define
> just like you do for -1.

Sure, I would use `if ( cpu_has_smep && opt_smep != SMEP_HVM_ONLY )` in
next version.
BTW. In v6 patch, I missed `cpu_has_smep` and that's why this patch is
buggy on the machine without smap/smep hardware feature.

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