KVM: PPC: Book3S PR: Enable MSR_DR for switch_mmu_context()

Commit 863771a28e27 ("powerpc/32s: Convert switch_mmu_context() to C")
moved the switch_mmu_context() to C. While in principle a good idea, it
meant that the function now uses the stack. The stack is not accessible
from real mode though.

So to keep calling the function, let's turn on MSR_DR while we call it.
That way, all pointer references to the stack are handled virtually.

In addition, make sure to save/restore r12 on the stack, as it may get
clobbered by the C function.

Fixes: 863771a28e27 ("powerpc/32s: Convert switch_mmu_context() to C")
Cc: stable@vger.kernel.org # v5.14+
Reported-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Alexander Graf <graf@amazon.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220510123717.24508-1-graf@amazon.com

diff --git a/arch/powerpc/kvm/book3s_32_sr.S b/arch/powerpc/kvm/book3s_32_sr.S
index e3ab9df..6cfcd20 100644 (file)
--- a/arch/powerpc/kvm/book3s_32_sr.S
+++ b/arch/powerpc/kvm/book3s_32_sr.S
@@ -122,11 +122,27 @@
 
        /* 0x0 - 0xb */
 
-       /* 'current->mm' needs to be in r4 */
-       tophys(r4, r2)
-       lwz     r4, MM(r4)
-       tophys(r4, r4)
-       /* This only clobbers r0, r3, r4 and r5 */
+       /* switch_mmu_context() needs paging, let's enable it */
+       mfmsr   r9
+       ori     r11, r9, MSR_DR
+       mtmsr   r11
+       sync
+
+       /* switch_mmu_context() clobbers r12, rescue it */
+       SAVE_GPR(12, r1)
+
+       /* Calling switch_mmu_context(<inv>, current->mm, <inv>); */
+       lwz     r4, MM(r2)
        bl      switch_mmu_context
 
+       /* restore r12 */
+       REST_GPR(12, r1)
+
+       /* Disable paging again */
+       mfmsr   r9
+       li      r6, MSR_DR
+       andc    r9, r9, r6
+       mtmsr   r9
+       sync
+
 .endm