Re: [RFC][PATCH 01/34] x86/xen: Explain why calling get_cpu_cap() so early is a hack

Borislav Petkov
2024-03-07T16:28:11Z

On Thu, Feb 22, 2024 at 10:39:28AM -0800, Dave Hansen wrote:
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> xen_start_kernel() some of the first C code run "Xen PV" systems.

That sentence reads weird.

> It precedes normally very early things like setup_arch() or the
> processor initialization code.
>
> That means that 'boot_cpu_data' is garbage. It has not even

s/is/contains/

> established the utter basics like if the CPU supports the CPUID
> instruction. Unfortunately get_cpu_cap() requires this exact
> information.
>
> Nevertheless xen_start_kernel() calls get_cpu_cap(). But it
> works out in practice because it's looking for the NX bit which
> comes from an extended CPUID leaf that doesn't depend on
> c->cpuid_level being set. This also implicitly assumes that
> Xen PV guests support CPUID.
>
> Leave the hack in place, but at least explain some of what is
> going on.
>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Juergen Gross <jgross@suse.com>
> ---
>
> b/arch/x86/xen/enlighten_pv.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff -puN arch/x86/xen/enlighten_pv.c~xen-explain1 arch/x86/xen/enlighten_pv.c
> --- a/arch/x86/xen/enlighten_pv.c~xen-explain1 2024-02-22 10:08:48.404451146 -0800
> +++ b/arch/x86/xen/enlighten_pv.c 2024-02-22 10:08:48.404451146 -0800
> @@ -1372,7 +1372,11 @@ asmlinkage __visible void __init xen_sta
> /* Get mfn list */
> xen_build_dynamic_phys_to_machine();
>
> - /* Work out if we support NX */
> + /*
> + * This is a hack. 'boot_cpu_data' is not filled out enough
> + * for get_cpu_cap() to function fully. But it _can_ fill out
> + * the leaves where NX is. Gross, but it works.
> + */
> get_cpu_cap(&boot_cpu_data);
> x86_configure_nx();

If all it needs is to figure out whether it supports NX, just do that by
foot here and get rid of the get_cpu_cap(&boot_cpu_data) call
completely.

It won't be a hack anymore.

Thx.

--
Regards/Gruss,
Boris.