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.
Reported-by: Matt Evans <[email protected]>
Fixes: 863771a28e27 ("powerpc/32s: Convert switch_mmu_context() to C")
Signed-off-by: Alexander Graf <[email protected]>
Cc: [email protected] # v5.14+
---
v1 -> v2:
- Save and restore R12, so that we don't touch volatile registers
while calling into C.
v2 -> v3:
- Save and restore R12 on the stack. SPRGs may be clobbered by
page faults.
---
arch/powerpc/kvm/book3s_32_sr.S | 26 +++++++++++++++++++++-----
1 file changed, 21 insertions(+), 5 deletions(-)
diff --git a/arch/powerpc/kvm/book3s_32_sr.S b/arch/powerpc/kvm/book3s_32_sr.S
index e3ab9df6cf19..6cfcd20d4668 100644
--- 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
--
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Le 10/05/2022 à 14:37, Alexander Graf a écrit :
> 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.
>
> Reported-by: Matt Evans <[email protected]>
> Fixes: 863771a28e27 ("powerpc/32s: Convert switch_mmu_context() to C")
> Signed-off-by: Alexander Graf <[email protected]>
> Cc: [email protected] # v5.14+
>
> ---
>
> v1 -> v2:
>
> - Save and restore R12, so that we don't touch volatile registers
> while calling into C.
>
> v2 -> v3:
>
> - Save and restore R12 on the stack. SPRGs may be clobbered by
> page faults.
> ---
> arch/powerpc/kvm/book3s_32_sr.S | 26 +++++++++++++++++++++-----
> 1 file changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_32_sr.S b/arch/powerpc/kvm/book3s_32_sr.S
> index e3ab9df6cf19..6cfcd20d4668 100644
> --- 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
Instead of li/andc you can do:
rlwinm r9, r9, 0, ~MSR_DR
> + mtmsr r9
> + sync
> +
> .endm
On Tue, 10 May 2022 14:37:17 +0200, Alexander Graf wrote:
> 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.
>
> [...]
Applied to powerpc/fixes.
[1/1] KVM: PPC: Book3S PR: Enable MSR_DR for switch_mmu_context()
https://git.kernel.org/powerpc/c/ee8348496c77e3737d0a6cda307a521f2cff954f
cheers