2022-05-10 14:43:22

by Alexander Graf

[permalink] [raw]
Subject: [PATCH v2] KVM: PPC: Book3S PR: Enable MSR_DR for switch_mmu_context()

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 in an SPRG, as it may get
clobbered by the C function.

Reported-by: Matt Evans <[email protected]>
Fixes: 863771a28e27 ("powerpc/32s: Convert switch_mmu_context() to C")
Signed-off-by: Alexander Graf <[email protected]>
Cc: [email protected] # 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
--
2.28.0.394.ge197136389




Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879





2022-05-10 15:15:31

by Alexander Graf

[permalink] [raw]
Subject: Re: [PATCH v2] KVM: PPC: Book3S PR: Enable MSR_DR for switch_mmu_context()


On 10.05.22 13:31, Christophe Leroy wrote:
> 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 ?


A DSI itself will be an interrupt again which will in turn destroy the
SPRG that we're saving. Guess I was trying to be too smart :). I'll use
Matt's original suggestion and just put it on the stack.


>> In addition, make sure to save/restore r12 in an SPRG, as it may get
>> clobbered by the C function.
>>
>> Reported-by: Matt Evans <[email protected]>
>> 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().


No worries, the compiled C version looks a lot nicer than the previous
asm one - and it was a good way to identify whether there still are
users of KVM on Book3S 32bit out there :)


Alex





Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879


2022-05-10 15:50:16

by Christophe Leroy

[permalink] [raw]
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 <[email protected]>
> 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 <[email protected]>
> Cc: [email protected] # 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