[virt-tools-list] [virt-manager PATCH] Allow installation of i686 guests on x86_64 machines

[Daniel P. Berrangé]

On Sat, Mar 03, 2018 at 04:21:37PM -0500, Cole Robinson wrote:
> On 03/03/2018 04:03 PM, Martin Kletzander wrote:
> > On Sat, Mar 03, 2018 at 03:53:59PM -0500, Cole Robinson wrote:
> >> On 03/01/2018 03:52 AM, Martin Kletzander wrote:
> >>> Do that by changing the code that was disabling it into code that
> >>> just warns in
> >>> such case.
> >>>
> >>> Signed-off-by: Martin Kletzander <mkletzan at redhat.com>
> >>> ---
> >>>  virtManager/create.py | 10 ++++------
> >>>  1 file changed, 4 insertions(+), 6 deletions(-)
> >>>
> >>> diff --git a/virtManager/create.py b/virtManager/create.py
> >>> index 0a73309372d9..9a40aec4a068 100644
> >>> --- a/virtManager/create.py
> >>> +++ b/virtManager/create.py
> >>> @@ -495,6 +495,10 @@ class vmmCreate(vmmGObjectUI):
> >>>                  msg = _("Failed to setup UEFI for AArch64: %s\n"
> >>>                          "Install options are limited.") % e
> >>>                  self._show_arch_warning(msg)
> >>> +        elif (self._capsinfo.arch == "i686" and
> >>> +              self.conn.caps.host.cpu.arch == "x86_64"):
> >>> +            msg = _("You are installing 32bit guest on 64bit host")
> >>> +            self._show_arch_warning(msg)
> >>>
> >>>          # Install Options
> >>>          method_tree = self.widget("method-tree")
> >>> @@ -824,12 +828,6 @@ class vmmCreate(vmmGObjectUI):
> >>>              if guest.os_type == self._capsinfo.os_type:
> >>>                  archs.append(guest.arch)
> >>>
> >>> -        # Combine x86/i686 to avoid confusion
> >>> -        if (self.conn.caps.host.cpu.arch == "x86_64" and
> >>> -            "x86_64" in archs and "i686" in archs):
> >>> -            archs.remove("i686")
> >>> -        archs.sort()
> >>> -
> >>>          prios = ["x86_64", "i686", "aarch64", "armv7l", "ppc64",
> >>> "ppc64le",
> >>>              "s390x"]
> >>>          if self.conn.caps.host.cpu.arch not in prios:
> >>>
> >>
> >> The idea behind hiding the option is 1) it should be rare that someone
> >> actually wants their VM to present a 32bit cpu on 64bit host, 2) hiding
> >> it means when only x86_64 qemu is installed we can entirely hide the
> >> 'advanced options' expander.
> >>
> >> Example: on rhel7 x86_64, libvirt advertises arch=i686 and arch=x86_64
> >> for /usr/libexec/qemu-kvm. If we hide i686, there's only one arch option
> >> available, and virt-manager will hide the entire 'advanced options'
> >> expander. This is ideal IMO, otherwise users might go clicking, see the
> >> i686 option, misinterpret it to mean OS arch (which in the context of
> >> virt I've seen people mistake many times), set things to i686 needlessly.
> >>
> > 
> > Sure I get that, that's why I kept it as a warning.
> > 
> >> Are there benefits I'm missng of doing arch=i686 on an x86_64 host
> >> exactly? Doesn't it just map to using qemu32 as the default CPU?
> >>
> > 
> > To be honest, I'm not sure if qemu32 CPU type means also 32bit CPU or
> > just limited instruction set and how it looks from the guest OS POV.
> > The reason for this patch emerged simply when we needed to test libvirt
> > build failure on 32bit machine and it was just easier to install 32bit
> > VM than setting up a cross-build.  Or rather installing all 32bit
> > dependencies properly.  I'd love to hear how to do this properly, I'm
> > not saying this patch is needed =)
> 
> I think this is an example of that arch confusion actually. You can
> install a 32 bit OS into a 64 bit machine, doesn't matter if the CPU is
> x86_64, and building in that should reproduce the 32bit build error

Yes, in fact all x86_64 CPUs start off as stupid 16-bit CPUs when first
powered on. The guest OS has to put them into 32-bit or 64-bit mode
themselves. So by installing a 32-bit OS you'll simply never see any
of the things that makes it capable of 64-bit.  If you really absolutely
want to block use of 64-bit mode though, you could do that by simply
telling libvirt to block the "lm" feature flag from the emulated CPU.
That'll stop the guest OS detecting availability of 64-bit and thus
it'll only be 32-bit.

Regards,
Daniel
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