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SubjectRe: [PATCH v2] KVM: PPC: Book3S PR: Enable MSR_DR for switch_mmu_context()
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Le 10/05/2022 à 13:18, 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.

Is the system ready to handle a DSI in case the stack is not mapped ?

>
> In addition, make sure to save/restore r12 in an SPRG, as it may get
> clobbered by the C function.
>
> Reported-by: Matt Evans <matt@ozlabs.org>
> Fixes: 863771a28e27 ("powerpc/32s: Convert switch_mmu_context() to C")

Oops, sorry for that. I didn't realise that there was other callers to
switch_mmu_context() than switch_mm_irqs_off().

Christophe

> Signed-off-by: Alexander Graf <graf@amazon.com>
> Cc: stable@vger.kernel.org # v5.14+
>
> ---
>
> v1 -> v2:
>
> - Save and restore R12, so that we don't touch volatile registers
> while calling into C.
> ---
> 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..1ce13e3ab072 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() clobbers r12, rescue it */
> + SET_SCRATCH0(r12)
> +
> + /* switch_mmu_context() needs paging, let's enable it */
> + mfmsr r9
> + ori r11, r9, MSR_DR
> + mtmsr r11
> + sync
> +
> + /* Calling switch_mmu_context(<inv>, current->mm, <inv>); */
> + lwz r4, MM(r2)
> bl switch_mmu_context
>
> + /* Disable paging again */
> + mfmsr r9
> + li r6, MSR_DR
> + andc r9, r9, r6
> + mtmsr r9
> + sync
> +
> + /* restore r12 */
> + GET_SCRATCH0(r12)
> +
> .endm
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