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@xxxxxxxxxxxxx>
---
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
