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[Xen-devel] [PATCH] xen/arm64: ensure that the correct SP is used for exceptions

From: "Kyle J. Temkin" <temkink@xxxxxxxxxxxx>

The ARMv8 architecture has a SPSel ("stack pointer selection") machine
register that allows us to determine which exception level's stack
pointer is loaded when an exception occurs. As we don't want to
use the non-priveleged SP_EL0 stack pointer -- or even assume that SP_EL0
points to a valid address in the hypervisor context--  we'll need to ensure
that our EL2 code sets the SPSel to SP_ELn mode, so exceptions that trap
to EL2 use the EL2 stack pointer.

This corrects an issue that can manifest as a hang-on-IRQ on some
arm64 cores if the firmware/bootloader has previously initialized SPSel
to 0; in which case Xen's exceptions will incorrectly use an invalid SP_EL0,
and will endlessly spin on the synchronous abort handler.

Signed-off-by: Kyle Temkin <temkink@xxxxxxxxxxxx>
 xen/arch/arm/arm64/head.S | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 946e2c9..d5831f2 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -361,6 +361,11 @@ skip_bss:
         ldr   x0, =(HSCTLR_BASE)
         msr   SCTLR_EL2, x0
+        /* Ensure that any exceptions encountered at EL2
+         * are handled using the EL2 stack pointer, rather
+         * than SP_EL0. */
+        msr spsel, #1
         /* Rebuild the boot pagetable's first-level entries. The structure
          * is described in mm.c.

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