[PATCH v4 1/3] ppc64/book3s: fix branching to out of line handlers in relocation kernel

[Hari Bathini]

Some of the interrupt vectors on 64-bit POWER server processors  are
only 32 bytes long (8 instructions), which is not enough for the full
first-level interrupt handler. For these we need to branch to an out-
of-line (OOL) handler. But when we are running a relocatable kernel,
interrupt vectors till __end_interrupts marker are copied down to real
address 0x100. So, branching to labels (read OOL handlers) outside this
section should be handled differently (see LOAD_HANDLER()), considering
relocatable kernel, which would need atleast 4 instructions.

However, branching from interrupt vector means that we corrupt the CFAR
(come-from address register) on POWER7 and later processors as mentioned
in commit 1707dd16. So, EXCEPTION_PROLOG_0 (6 instructions) that contains
the part up to the point where the CFAR is saved in the PACA should be
part of the short interrupt vectors before we branch out to OOL handlers.

But as mentioned already, there are interrupt vectors on 64-bit POWER server
processors that are only 32 bytes long (like vectors 0x4f00, 0x4f20, etc.),
which cannot accomodate the above two cases at the same time owing to space
constraint. Currently, in these interrupt vectors, we simply branch out to
OOL handlers, without using LOAD_HANDLER(), which leaves us vulnerable when
running a relocatable kernel (eg. kdump case). While this has been the case
for sometime now and kdump is used widely, we were fortunate not to see any
problems so far, for three reasons:

    1. In almost all cases, production kernel (relocatable) is used for
       kdump as well, which would mean that crashed kernel's OOL handler
       would be at the same place where we endup branching to, from short
       interrupt vector of kdump kernel.
    2. Also, OOL handler was unlikely the reason for crash in almost all
       the kdump scenarios, which meant we had a sane OOL handler from
       crashed kernel that we branched to.
    3. On most 64-bit POWER server processors, page size is large enough
       that marking interrupt vector code as executable (see commit
       429d2e83) leads to marking OOL handler code from crashed kernel,
       that sits right below interrupt vector code from kdump kernel, as
       executable as well.

Let us fix this undependable code path by moving these OOL handlers below
__end_interrupts marker to make sure we also copy these handlers to real
address 0x100 when running a relocatable kernel. Because the interrupt
vectors branching to these OOL handlers are not long enough to use
LOAD_HANDLER() for branching as discussed above.

This fix has been tested successfully in kdump scenario, on a lpar with 4K page
size by using different default/production kernel and kdump kernel.

Signed-off-by: Hari Bathini <hbathini at linux.vnet.ibm.com>
Signed-off-by: Mahesh Salgaonkar <mahesh at linux.vnet.ibm.com>
---

Michael, I did test this patchset in different scenarios. But if you feel
the change is too radical, we could go with version2. But I thought this was
worth a shot.

changes from v3:
1. No changes in this patch except for a spellcheck
2. A new patch that tries to free up space below 0x7000 (2/3)
3. A new patch to remove __end_handlers marker (3/3)


 arch/powerpc/kernel/exceptions-64s.S |   29 +++++++++++++++++++----------
 1 file changed, 19 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
index 7716ceb..f76b2f3 100644
--- a/arch/powerpc/kernel/exceptions-64s.S
+++ b/arch/powerpc/kernel/exceptions-64s.S
@@ -953,6 +953,25 @@ hv_facility_unavailable_relon_trampoline:
 #endif
 	STD_RELON_EXCEPTION_PSERIES(0x5700, 0x1700, altivec_assist)
 
+	/*
+	 * Out-Of-Line handlers for relocation-on interrupt vectors
+	 *
+	 * We need these OOL handlers to be below __end_interrupts
+	 * marker to ensure we also copy these OOL handlers along
+	 * with the interrupt vectors to real address 0x100 when
+	 * running a relocatable kernel. Because the interrupt
+	 * vectors branching to these OOL handlers are not long
+	 * enough to use LOAD_HANDLER() for branching.
+	 */
+	STD_RELON_EXCEPTION_HV_OOL(0xe40, emulation_assist)
+	MASKABLE_RELON_EXCEPTION_HV_OOL(0xe80, h_doorbell)
+
+	STD_RELON_EXCEPTION_PSERIES_OOL(0xf00, performance_monitor)
+	STD_RELON_EXCEPTION_PSERIES_OOL(0xf20, altivec_unavailable)
+	STD_RELON_EXCEPTION_PSERIES_OOL(0xf40, vsx_unavailable)
+	STD_RELON_EXCEPTION_PSERIES_OOL(0xf60, facility_unavailable)
+	STD_RELON_EXCEPTION_HV_OOL(0xf80, hv_facility_unavailable)
+
 	/* Other future vectors */
 	.align	7
 	.globl	__end_interrupts
@@ -1234,16 +1253,6 @@ END_FTR_SECTION_IFSET(CPU_FTR_VSX)
 	.globl	__end_handlers
 __end_handlers:
 
-	/* Equivalents to the above handlers for relocation-on interrupt vectors */
-	STD_RELON_EXCEPTION_HV_OOL(0xe40, emulation_assist)
-	MASKABLE_RELON_EXCEPTION_HV_OOL(0xe80, h_doorbell)
-
-	STD_RELON_EXCEPTION_PSERIES_OOL(0xf00, performance_monitor)
-	STD_RELON_EXCEPTION_PSERIES_OOL(0xf20, altivec_unavailable)
-	STD_RELON_EXCEPTION_PSERIES_OOL(0xf40, vsx_unavailable)
-	STD_RELON_EXCEPTION_PSERIES_OOL(0xf60, facility_unavailable)
-	STD_RELON_EXCEPTION_HV_OOL(0xf80, hv_facility_unavailable)
-
 #if defined(CONFIG_PPC_PSERIES) || defined(CONFIG_PPC_POWERNV)
 /*
  * Data area reserved for FWNMI option.