linux-kernel - Re: [PATCH v2] KVM: PPC: Book3S PR: Enable MSR_DR for switch_mmu

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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