Re: [Libguestfs] [PATCH] lib: Disable 5-level page tables when using -cpu max

On 05/12/22 09:52, Richard W.M. Jones wrote: > In https://bugzilla.redhat.com/show_bug.cgi?id=2082806 we've been > tracking an insidious qemu bug which intermittently prevents the > libguestfs appliance from starting. The symptoms are that SeaBIOS > starts and displays its messages, but the kernel isn't reached. We > found that the kernel does in fact start, but when it tries to set up > page tables and jump to protected mode it gets a triple fault which > causes the emulated CPU in qemu to reset (qemu exits). > > This seems to only affect TCG (not KVM). > > Yesterday I found that this is caused by using -cpu max which enables > the "la57" feature (5-level page tables[0]), and that we can make the > problem go away using -cpu max,la57=off. Note that I still don't > fully understand the qemu bug, so this is only a workaround. > > I chose to disable 5-level page tables for both TCG and KVM, partly to > make the patch simpler, and partly because I guess it's not a feature > (ie. 57 bit linear addresses) that is useful for the libguestfs > appliance case, where we have limited physical memory and no need to > run any programs with huge address spaces. > > I tested this by running both the direct & libvirt paths overnight. I > expect that this patch will fail with old qemu/libvirt which doesn't > understand the "la57" feature, but this is only intended as a > temporary workaround. > > [0] Article about 5-level page tables as background: > https://lwn.net/Articles/717293/ > > Thanks: Laszlo Ersek > Fixes: https://answers.launchpad.net/ubuntu/+source/libguestfs/+question/701625 > --- > lib/launch-direct.c | 15 +++++++++++++-- > lib/launch-libvirt.c | 7 +++++++ > 2 files changed, 20 insertions(+), 2 deletions(-) > > diff --git a/lib/launch-direct.c b/lib/launch-direct.c > index c07a8d78f..ff0eaeb62 100644 > --- a/lib/launch-direct.c > +++ b/lib/launch-direct.c > @@ -518,8 +518,19 @@ launch_direct (guestfs_h *g, void *datav, const char *arg) > } end_list (); > > cpu_model = guestfs_int_get_cpu_model (has_kvm && !force_tcg); > - if (cpu_model) > - arg ("-cpu", cpu_model); > + if (cpu_model) { > +#if defined(__x86_64__) > + /* Temporary workaround for RHBZ#2082806 */ > + if (STREQ (cpu_model, "max")) { > + start_list ("-cpu") { > + append_list (cpu_model); > + append_list ("la57=off"); > + } end_list (); > + } > + else > +#endif > + arg ("-cpu", cpu_model); > + } > > if (g->smp > 1) > arg_format ("-smp", "%d", g->smp); > diff --git a/lib/launch-libvirt.c b/lib/launch-libvirt.c > index 87da2f40e..03d69e027 100644 > --- a/lib/launch-libvirt.c > +++ b/lib/launch-libvirt.c > @@ -1185,6 +1185,13 @@ construct_libvirt_xml_cpu (guestfs_h *g, > else if (STREQ (cpu_model, "max")) { > /* https://bugzilla.redhat.com/show_bug.cgi?id=1935572#c11 */ > attribute ("mode", "maximum"); > +#if defined(__x86_64__) > + /* Temporary workaround for RHBZ#2082806 */ > + start_element ("feature") { > + attribute ("policy", "disable"); > + attribute ("name", "la57"); > + } end_element (); > +#endif > } > else > single_element ("model", cpu_model); > Acked-by: Laszlo Ersek <lersek(a)redhat.com&gt;