[XEN PATCH] x86/ACPI: Ignore entries marked as unusable when parsing MADT

[Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>]

Up to version 6.2 Errata B [2] the ACPI spec only defines
ACPI_MADT_ENABLE as:

    If zero, this processor is unusable, and the operating system
    support will not attempt to use it.

The bit that later will be ACPI_MADT_ONLINE_CAPABLE is reserved with
"Must be zero".

Version 6.3 [3] then adds ACPI_MADT_ONLINE_CAPABLE and changes the
meaning of ACPI_MADT_ENABLE:

    Enabled
        If this bit is set the processor is ready for use. If this bit
        is clear and the Online Capable bit is set, system hardware
        supports enabling this processor during OS runtime. If this bit
        is clear and the Online Capable bit is also clear, this
        processor is unusable, and OSPM shall ignore the contents of the
        Processor Local APIC Structure.

    Online Capbable
        The information conveyed by this bit depends on the value of the
        Enabled bit. If the Enabled bit is set, this bit is reserved and
        must be zero. Otherwise, if this this bit is set, system
        hardware supports enabling this processor during OS runtime.

So with conforming firmwares it should be safe to simply ignore the
entry if !ACPI_MADT_ENABLED && !ACPI_MADT_ONLINE_CAPABLE

As a precaution against buggy firmwares this change, like Linux [4],
ignores ACPI_MADT_ONLINE_CAPABLE completely if MADT revision < 5. Note
that the MADT revision was already increased to 5 with spec version 6.2
Errata A [1], so before introducing the online capable flag. But it
wasn't changed for the new flag, so this is the best we can do here.

For previous discussion see thread [5].

Link: http://www.uefi.org/sites/default/files/resources/ACPI%206_2_A_Sept29.pdf 
# [1]
Link: https://uefi.org/sites/default/files/resources/ACPI_6_2_B_final_Jan30.pdf 
# [2]
Link: https://uefi.org/sites/default/files/resources/ACPI_6_3_May16.pdf # [3]
Link: 
https://git.kernel.org/torvalds/c/e2869bd7af608c343988429ceb1c2fe99644a01f # [4]
Link: 
https://lore.kernel.org/xen-devel/80bae614-052e-0f90-cf13-0e5e4ed1a5cd@xxxxxxxxxxxxxxxxxxxxxx/
 # [5]
Signed-off-by: Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>
---
 xen/arch/x86/acpi/boot.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/acpi/boot.c b/xen/arch/x86/acpi/boot.c
index 4a62822fa9..2d0b8a9afc 100644
--- a/xen/arch/x86/acpi/boot.c
+++ b/xen/arch/x86/acpi/boot.c
@@ -77,6 +77,17 @@ static int __init cf_check acpi_parse_madt(struct 
acpi_table_header *table)
        return 0;
 }
 
+static bool __init acpi_is_processor_usable(uint32_t lapic_flags)
+{
+       if (lapic_flags & ACPI_MADT_ENABLED)
+               return true;
+
+       if (madt_revision >= 5 && (lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
+               return true;
+
+       return false;
+}
+
 static int __init cf_check
 acpi_parse_x2apic(struct acpi_subtable_header *header, const unsigned long end)
 {
@@ -92,9 +103,7 @@ acpi_parse_x2apic(struct acpi_subtable_header *header, const 
unsigned long end)
                return 0;
 
        /* Don't register processors that cannot be onlined. */
-       if (madt_revision >= 5 &&
-           !(processor->lapic_flags & ACPI_MADT_ENABLED) &&
-           !(processor->lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
+       if (!acpi_is_processor_usable(processor->lapic_flags))
                return 0;
 
        if ((processor->lapic_flags & ACPI_MADT_ENABLED) || opt_cpu_info) {
@@ -151,9 +160,7 @@ acpi_parse_lapic(struct acpi_subtable_header * header, 
const unsigned long end)
                return 0;
 
        /* Don't register processors that cannot be onlined. */
-       if (madt_revision >= 5 &&
-           !(processor->lapic_flags & ACPI_MADT_ENABLED) &&
-           !(processor->lapic_flags & ACPI_MADT_ONLINE_CAPABLE))
+       if (!acpi_is_processor_usable(processor->lapic_flags))
                return 0;
 
        if ((processor->lapic_flags & ACPI_MADT_ENABLED) || opt_cpu_info)
-- 
2.40.1