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

[Laszlo Ersek]

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>