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Message-ID: <f7416897-2ca5-6b2f-cfd3-30d9bcc557cd@csgroup.eu>
Date: Tue, 10 May 2022 11:31:30 +0000
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Alexander Graf <graf@...zon.com>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>
CC: "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Paul Mackerras <paulus@...ba.org>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Michael Ellerman <mpe@...erman.id.au>,
Matt Evans <matt@...abs.org>,
"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH v2] KVM: PPC: Book3S PR: Enable MSR_DR for
switch_mmu_context()
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@...abs.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@...zon.com>
> Cc: stable@...r.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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