Re: [PATCH] x86/static-call: Remove early_boot_irqs_disabled check to fix Xen PVH dom0

[Jürgen Groß]

On 21.12.24 22:10, Andrew Cooper wrote:
> __static_call_update_early() has a check for early_boot_irqs_disabled, but
> is used before early_boot_irqs_disabled is set up in start_kernel().
>
> Xen PV has always special cased early_boot_irqs_disabled, but Xen PVH does
> not and falls over the BUG when booting as dom0.
>
> It is very suspect that early_boot_irqs_disabled starts as 0, becomes 1 for
> a time, then becomes 0 again, but as this needs backporting to fix a
> breakage in a security fix, dropping the BUG_ON() is the far safer option.
>
> Fixes: 0ef8047b737d ("x86/static-call: provide a way to do very early static-call updates")
> Reported-by: Alex Zenla <alex@xxxxxxxxx>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219620
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Tested-by: Alex Zenla <alex@xxxxxxxxx>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> CC: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> CC: Jason Baron <jbaron@xxxxxxxxxx>
> CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
> CC: Ard Biesheuvel <ardb@xxxxxxxxxx>
> CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CC: Ingo Molnar <mingo@xxxxxxxxxx>
> CC: Borislav Petkov <bp@xxxxxxxxx>
> CC: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> CC: x86@xxxxxxxxxx
> CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
> CC: Juergen Gross <jgross@xxxxxxxx>
> CC: linux-kernel@xxxxxxxxxxxxxxx
> CC: stable@xxxxxxxxxx

Reviewed-by: Juergen Gross <jgross@xxxxxxxx>

> It's not entirely clear why PVH domU is fine but PVH dom0 is not.  It crashes
> so early there's no console or useful backtrace.

I suspect that the Xen hypervisor doesn't supply a memory map for PVH dom0 via
the start_info data, while the Xen tools do so for PVH guests. This requires
dom0 to issue a hypercall very early in order to obtain the memory map.


Juergen

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