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

[Richard W.M. Jones]

On Fri, May 13, 2022 at 12:59:39PM +0200, Laszlo Ersek wrote:
> 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 at redhat.com>

I pushed this as commit 59d7e6e017.

FYI these bugs:
https://bugzilla.redhat.com/show_bug.cgi?id=2084566
https://bugzilla.redhat.com/show_bug.cgi?id=2084567
https://bugzilla.redhat.com/show_bug.cgi?id=2084568

Rich.

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