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Subject[PATCH v2 01/12] x86/apic: Install an empty physflat_init_apic_ldr
Date
From: Jan Kiszka <jan.kiszka@siemens.com>

As the comment already stated, there is no need for setting up LDR (and
DFR) in physflat mode as it remains unused (see SDM, 10.6.2.1).
flat_init_apic_ldr only served as a placeholder for a nop operation so
far, causing no harm.

That will change when running over the Jailhouse hypervisor. Here we
must not touch LDR in a way that destroys the mapping originally set up
by the Linux root cell. Jailhouse enforces this setting in order to
efficiently validate any IPI requests sent by a cell.

Avoid a needless clash caused by flat_init_apic_ldr by installing a true
nop handler.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---
arch/x86/kernel/apic/apic_flat_64.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/apic/apic_flat_64.c b/arch/x86/kernel/apic/apic_flat_64.c
index aa85690e9b64..226a6ab22b35 100644
--- a/arch/x86/kernel/apic/apic_flat_64.c
+++ b/arch/x86/kernel/apic/apic_flat_64.c
@@ -218,6 +218,15 @@ static int physflat_acpi_madt_oem_check(char *oem_id, char *oem_table_id)
return 0;
}

+static void physflat_init_apic_ldr(void)
+{
+ /*
+ * LDR and DFR are not involved in physflat mode, rather:
+ * "In physical destination mode, the destination processor is
+ * specified by its local APIC ID [...]." (Intel SDM, 10.6.2.1)
+ */
+}
+
static void physflat_send_IPI_allbutself(int vector)
{
default_send_IPI_mask_allbutself_phys(cpu_online_mask, vector);
@@ -251,8 +260,7 @@ static struct apic apic_physflat __ro_after_init = {
.dest_logical = 0,
.check_apicid_used = NULL,

- /* not needed, but shouldn't hurt: */
- .init_apic_ldr = flat_init_apic_ldr,
+ .init_apic_ldr = physflat_init_apic_ldr,

.ioapic_phys_id_map = NULL,
.setup_apic_routing = NULL,
--
2.12.3
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