Hi Alex,

On 9 May 2022, at 21:23, Alexander Graf graf@amazon.com 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.

Reported-by: Matt Evans matt@ozlabs.org Fixes: 863771a28e27 ("powerpc/32s: Convert switch_mmu_context() to C") Signed-off-by: Alexander Graf graf@amazon.com Cc: stable@vger.kernel.org

Many thanks - this addresses the issue I saw, and has been...

Tested-by: Matt Evans matt@ozlabs.org

...on a G4 host. One comment though:

— arch/powerpc/kvm/book3s_32_sr.S | 20 +++++++++++++++----- 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_32_sr.S b/arch/powerpc/kvm/book3s_32_sr.S index e3ab9df6cf19..bd4f798f7a46 100644 --- a/arch/powerpc/kvm/book3s_32_sr.S +++ b/arch/powerpc/kvm/book3s_32_sr.S @@ -122,11 +122,21 @@

/* 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
• /* Calling switch_mmu_context(<inv>, current->mm, <inv>); */
• lwz r4, MM(r2) bl switch_mmu_context

Of the volatile registers, I believe r12 is still valuable here and would need to be preserved. (I can’t spot any others but would defer to your judgement here.)

For example:

diff --git a/arch/powerpc/kvm/book3s_32_sr.S b/arch/powerpc/kvm/book3s_32_sr.S index e3ab9df6cf19..41fc9ca12d38 100644 --- a/arch/powerpc/kvm/book3s_32_sr.S +++ b/arch/powerpc/kvm/book3s_32_sr.S @@ -122,11 +122,23 @@

/* 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 + + SAVE_GPR(12, r1) + /* Calling switch_mmu_context(<inv>, current->mm, <inv>); */ + lwz r4, MM(r2) bl switch_mmu_context + REST_GPR(12, r1) + + /* Disable paging again */ + mfmsr r9 + li r6, MSR_DR + andc r9, r9, r6 + mtmsr r9 + sync

.endm

Matt

• /* Disable paging again */
• mfmsr r9
• li r6, MSR_DR
• andc r9, r9, r6
• mtmsr r9
• sync

.endm

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