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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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