Am 22.12.2010 13:33, 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| -no-kvm
> ----------------+-----------+-----------+----------
> KVM available | enabled* | enabled | disabled
> KVM unavailable | disabled | 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).
>
> Eventually, we'll sort this upstream with -accel (defaults tied to
> machine type). Until then, this patch reduces the difference to
> upstream so that most users shouldn't need to be aware of them.
>
> Make -enable-kvm behave just like in upstream: enable KVM, fail if not
> available. But retain current default behavior: enable KVM, fall back
> to TCG.
>
> qemu-kvm new | default |-enable-kvm| -no-kvm
> ----------------+-----------+-----------+-----------
> KVM available | enabled* | enabled | disabled
> KVM unavailable | disabled | fail+ | disabled
>
> * differs from upstream
> + changed by this patch
>
> Bonus fix: -no-kvm -enable-kvm now enables KVM. Before, it disabled it.
>
> Signed-off-by: Markus Armbruster <armbru@xxxxxxxxxx>
> ---
> vl.c | 22 +++++++++-------------
> 1 files changed, 9 insertions(+), 13 deletions(-)
>
> diff --git a/vl.c b/vl.c
> index e3c8919..1958e01 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 = -1;
> 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;
> @@ -2789,19 +2787,17 @@ 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));
> + if (kvm_allowed > 0) {
> + if (!kvm_available()) {
> + printf("KVM not supported for this target\n");
> + } else {
> + fprintf(stderr, "failed to initialize KVM: %s\n", strerror(-ret));
> + }
> + exit(1);
> }
> - exit(1);
> -#endif
> -#ifdef CONFIG_KVM
> fprintf(stderr, "Could not initialize KVM, will disable KVM support\n");
> - kvm_allowed = 0;
> -#endif
> }
> + kvm_allowed = ret >= 0;
> }
>
> if (qemu_init_main_loop()) {
Does any qemu-kvm user rely on the automatic fallback to TCG if KVM
initialization fails? If not, then just set kvm_allowed to 1 in qemu-kvm
and leave the rest as upstream provides it. This fallback is really
annoying, specifically as the only point of qemu-kvm is, well, running
over KVM.
Jan
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