[PATCH] x86/apic/flat64: Add back the early_param("apic", parse_apic)

[Dexuan Cui <decui@microsoft.com>]

parse_apic() allows the user to try a different apic driver than the
default one that's automatically chosen. It works for x86_32, but
doesn't work for x86_64 becauase it was removed in 2009 for x86_64 by:
commit 7b38725318f4 ("x86: remove subarchitecture support code"),
whose changelog doesn't explicitly describe the removal for x86_64.

The patch adds back the functionality for x86_64. The intent is mainly
to work around an APIC emulation bug in Hyper-V in the case of kdump:
currently Hyper-V does not honor the disabled state of the local APICs,
and all the IOAPIC-based interrupts may not be delivered to the correct
virtual CPU, if the logical-mode APIC driver is used (the kdump
kernel usually uses the logical-mode APIC driver, since typically
only 1 CPU is active). Luckily the kdump issue can be worked around by
forcing the kdump kernel to use physical mode, before the fix to Hyper-V
becomes widely available.

IMHO the patch is safe because the current default algorithm to choose
the apic driver is unchanged; the patch makes a difference only when
the user specifies the apic= kernel parameter, e.g. "apic=physical flat".

Signed-off-by: Dexuan Cui <decui@microsoft.com>
---
 arch/x86/kernel/apic/apic_flat_64.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/apic/apic_flat_64.c b/arch/x86/kernel/apic/apic_flat_64.c
index 7862b152a052..efbec63bb01f 100644
--- a/arch/x86/kernel/apic/apic_flat_64.c
+++ b/arch/x86/kernel/apic/apic_flat_64.c
@@ -23,9 +23,34 @@ static struct apic apic_flat;
 struct apic *apic __ro_after_init = &apic_flat;
 EXPORT_SYMBOL_GPL(apic);
 
+static int cmdline_apic __initdata;
+static int __init parse_apic(char *arg)
+{
+	struct apic **drv;
+
+	if (!arg)
+		return -EINVAL;
+
+	for (drv = __apicdrivers; drv < __apicdrivers_end; drv++) {
+		if (!strcmp((*drv)->name, arg)) {
+			apic = *drv;
+			cmdline_apic = 1;
+			return 0;
+		}
+	}
+
+	/* Parsed again by __setup for debug/verbose */
+	return 0;
+}
+early_param("apic", parse_apic);
+
+
 static int flat_acpi_madt_oem_check(char *oem_id, char *oem_table_id)
 {
-	return 1;
+	if (!cmdline_apic)
+		return 1;
+
+	return apic == &apic_flat;
 }
 
 /*
-- 
2.25.1