On 12/15/2010 09:50 AM, Markus Armbruster wrote:
We currently enable KVM by default, and when it's not available, we
print a message and fall back to TCG. Option -enable-kvm is ignored.
Option -no-kvm suppresses KVM.
Upstream works differently: KVM is off by default, -enable-kvm
switches it on. -enable-kvm terminates the process unsuccessfully if
KVM is not available.
upstream qemu | default |-enable-kvm
----------------+-----------+-----------
KVM available | disabled | enabled
KVM unavailable | disabled | fail
qemu-kvm | default |-enable-kvm
----------------+-----------+-----------
KVM available | enabled* | enabled
KVM unavailable | disabled | disabled*
* differs from upstream
Users of qemu and qemu-kvm need to be aware of these differences to
enable / disable use of KVM reliably. This is bothersome.
Consider -enable-kvm when KVM is unavailable: If the user expects
qemu-kvm behavior (fall back), but qemu fails, he'll likely be
surprised and unhappy. If the user expects upstream behavior (fail),
but qemu-kvm falls back to TCG, the guest runs slow as molasses, and
the user will likely be confused and unhappy (unless he spots and
understands the "disable KVM" message).
Switch to upstream semantics: KVM off by default, -enable-kvm switches
it on, and when it can't, it's fatal.
Having to enable KVM explicitly is annoying, but the proper place to
address that is upstream.
Signed-off-by: Markus Armbruster<armbru@xxxxxxxxxx>
Backwards compatibility is going to kill us if we try to make this change.
Current qemu-kvm behavior:
default: -accel kvm,tcg
-no-kvm: -accel tcg
-enable-kvm: -accel kvm,tcg
Current upstream behavior
default: -accel tcg
-enable-kvm: -accel kvm
I think we should tie `-accel' to the machine type. For qemu-kvm, a
different default machine type should be used than upstream qemu (it
really should be a configure switch).
For `pc', the default `-accel' behavior should remain 'tcg'. For
`kvmpc', the default `-accel' behavior should be 'kvm,tcg'.
-no-kvm should be deprecated. -enable-kvm should also be deprecated in
favor of the `-accel' option.
In the short term, it would be a good idea to modify qemu-kvm to switch
the -enable-kvm semantics to match upstream (fail if KVM isn't
available). Adding an alias for 'kvmpc' upstream and qemu-kvm and
making qemu-kvm default to 'kvmpc' would be helpful for management tools
too.
Regards,
Anthony Liguori
---
vl.c | 10 +---------
1 files changed, 1 insertions(+), 9 deletions(-)
diff --git a/vl.c b/vl.c
index e3c8919..87e88c2 100644
--- a/vl.c
+++ b/vl.c
@@ -247,7 +247,7 @@ static void *boot_set_opaque;
static NotifierList exit_notifiers =
NOTIFIER_LIST_INITIALIZER(exit_notifiers);
-int kvm_allowed = 1;
+int kvm_allowed = 0;
uint32_t xen_domid;
enum xen_mode xen_mode = XEN_EMULATE;
@@ -2436,10 +2436,8 @@ int main(int argc, char **argv, char **envp)
case QEMU_OPTION_smbios:
do_smbios_option(optarg);
break;
-#ifdef OBSOLETE_KVM_IMPL
case QEMU_OPTION_enable_kvm:
kvm_allowed = 1;
-#endif
break;
case QEMU_OPTION_no_kvm:
kvm_allowed = 0;
@@ -2789,18 +2787,12 @@ int main(int argc, char **argv, char **envp)
if (kvm_allowed) {
int ret = kvm_init(smp_cpus);
if (ret< 0) {
-#if defined(OBSOLETE_KVM_IMPL) || defined(CONFIG_NO_CPU_EMULATION)
if (!kvm_available()) {
printf("KVM not supported for this target\n");
} else {
fprintf(stderr, "failed to initialize KVM: %s\n", strerror(-ret));
}
exit(1);
-#endif
-#ifdef CONFIG_KVM
- fprintf(stderr, "Could not initialize KVM, will disable KVM support\n");
- kvm_allowed = 0;
-#endif
}
}
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