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 <matt@xxxxxxxxxx>
> Fixes: 863771a28e27 ("powerpc/32s: Convert switch_mmu_context() to C")
> Signed-off-by: Alexander Graf <graf@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # 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
