[libvirt] [PATCH 3/3] qemu: caps: Advertise arm 32-on-64 KVM option

[Daniel P. Berrange]

On Sat, Jun 06, 2015 at 07:06:42PM -0400, Cole Robinson wrote:
> On 05/28/2015 07:31 AM, Daniel P. Berrange wrote:
> > On Thu, May 21, 2015 at 07:03:28PM -0400, Cole Robinson wrote:
> >> We need to use qemu-system-aarch64 to run armv7l KVM VMs on an aarch64
> >> host.
> >> ---
> >>  src/qemu/qemu_capabilities.c | 42 ++++++++++++++++++++++--------------------
> >>  1 file changed, 22 insertions(+), 20 deletions(-)
> >>
> >> diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
> >> index 1e7bddb..7181865 100644
> >> --- a/src/qemu/qemu_capabilities.c
> >> +++ b/src/qemu/qemu_capabilities.c
> >> @@ -723,19 +723,6 @@ virQEMUCapsFindBinaryForArch(virArch hostarch,
> >>      return ret;
> >>  }
> >>  
> >> -
> >> -static bool
> >> -virQEMUCapsIsValidForKVM(virArch hostarch,
> >> -                         virArch guestarch)
> >> -{
> >> -    if (hostarch == guestarch)
> >> -        return true;
> >> -    if (hostarch == VIR_ARCH_X86_64 &&
> >> -        guestarch == VIR_ARCH_I686)
> >> -        return true;
> >> -    return false;
> >> -}
> >> -
> >>  static int
> >>  virQEMUCapsInitGuest(virCapsPtr caps,
> >>                       virQEMUCapsCachePtr cache,
> >> @@ -747,6 +734,7 @@ virQEMUCapsInitGuest(virCapsPtr caps,
> >>      char *binary = NULL;
> >>      virQEMUCapsPtr qemubinCaps = NULL;
> >>      virQEMUCapsPtr kvmbinCaps = NULL;
> >> +    bool native_kvm, x86_32on64_kvm, arm_32on64_kvm;
> >>      int ret = -1;
> >>  
> >>      /* Check for existence of base emulator, or alternate base
> >> @@ -764,16 +752,30 @@ virQEMUCapsInitGuest(virCapsPtr caps,
> >>  
> >>      /* qemu-kvm/kvm binaries can only be used if
> >>       *  - host & guest arches match
> >> -     * Or
> >> -     *  - hostarch is x86_64 and guest arch is i686
> >> -     * The latter simply needs "-cpu qemu32"
> >> +     *  - hostarch is x86_64 and guest arch is i686 (needs -cpu qemu32)
> >> +     *  - hostarch is aarch64 and guest arch is armv7l (needs -cpu aarch64=off)
> >>       */
> >> -    if (virQEMUCapsIsValidForKVM(hostarch, guestarch)) {
> >> -        const char *const kvmbins[] = { "/usr/libexec/qemu-kvm", /* RHEL */
> >> -                                        "qemu-kvm", /* Fedora */
> >> -                                        "kvm" }; /* Upstream .spec */
> >> +    native_kvm = (hostarch == guestarch);
> >> +    x86_32on64_kvm = (hostarch == VIR_ARCH_X86_64 &&
> >> +        guestarch == VIR_ARCH_I686);
> >> +    arm_32on64_kvm = (hostarch == VIR_ARCH_AARCH64 &&
> >> +        guestarch == VIR_ARCH_ARMV7L);
> >> +
> >> +    if (native_kvm || x86_32on64_kvm || arm_32on64_kvm) {
> >> +        const char *kvmbins[] = {
> >> +            "/usr/libexec/qemu-kvm", /* RHEL */
> >> +            "qemu-kvm", /* Fedora */
> >> +            "kvm", /* Debian/Ubuntu */
> >> +            NULL,
> >> +        };
> >> +
> >> +        if (arm_32on64_kvm)
> >> +            kvmbins[3] = "qemu-system-aarch64";
> > 
> > I'm unclear why you need to be adding this. We don't need it for
> > the equivalent i686 with qemu-system-x86_64, as the earlier call
> > to virQEMUCapsFindBinaryForArch() will already return the binary
> > qemu-system-x86_64. IIUC, it should have returned the binary
> > qemu-system-aarch64 too, so this just seems to duplicate the
> > check for that binary.
> 
> We need this in the case you are running on an aarch64 host and have both
> qemu-system-arm and qemu-system-aarch64 installed. In this case, when you want
> to use KVM for arm32, you _have_ to use qemu-system-aarch64, qemu-system-arm
> does not work. What you suggest would mean that qemu-system-arm is grabbed
> from the caps cache.
> 
> x86 doesn't have this problem because qemu-system-i386, qemu-system-x86_64 and
> by extension qemu-kvm can all be used to do 32-on-64 KVM.

Ok, I think we need to have this explained in the code comments, because
that is none obvious from reading the code & we don't want to be wondering
why we did this when looking at the code again in a year :-)

Regards,
Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|