Re: [PATCH v2] powerpc/rtas: Keep MSR[RI] set when calling RTAS

[Michael Ellerman]

Laurent Dufour <ldufour@xxxxxxxxxxxxx> writes:
> On 03/05/2022, 18:16:29, Fabiano Rosas wrote:
>> Michael Ellerman <mpe@xxxxxxxxxxxxxx> writes:
>> 
>>>> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
>>>> index 9581906b5ee9..65cb14b56f8d 100644
>>>> --- a/arch/powerpc/kernel/entry_64.S
>>>> +++ b/arch/powerpc/kernel/entry_64.S
>>>> @@ -330,22 +330,18 @@ _GLOBAL(enter_rtas)
>>>>  	clrldi	r4,r4,2			/* convert to realmode address */
>>>>         	mtlr	r4
>>>>  
>>>> -	li	r0,0
>>>> -	ori	r0,r0,MSR_EE|MSR_SE|MSR_BE|MSR_RI
>>>> -	andc	r0,r6,r0
>>>> -	
>>>> -        li      r9,1
>>>> -        rldicr  r9,r9,MSR_SF_LG,(63-MSR_SF_LG)
>>>> -	ori	r9,r9,MSR_IR|MSR_DR|MSR_FE0|MSR_FE1|MSR_FP|MSR_RI|MSR_LE
>>>> -	andc	r6,r0,r9
>>>  
>>> One advantage of the old method is it can adapt to new MSR bits being
>>> set by the kernel.
>>>
>>> For example we used to use RTAS on powernv, and this code didn't need
>>> updating to cater to MSR_HV being set. We will probably never use RTAS
>>> on bare-metal again, so that's OK.
>>>
>>> But your change might break secure virtual machines, because it clears
>>> MSR_S whereas the old code didn't. I think SVMs did use RTAS, but I
>>> don't know whether it matters if it's called with MSR_S set or not?
>>>
>>> Not sure if anyone will remember, or has a working setup they can test.
>>> Maybe for now we just copy MSR_S from the kernel MSR the way the
>>> current code does.
>> 
>> Would the kernel even be able to change the bit? I think only urfid can
>> clear MSR_S.
>
> That's a good point, thanks Fabiano!
>
> The POWER ISA programming note about MSR[S] is explicit:
> "MSR[S] can be set to 1 only by the System Call instruction and some
> interrupts. It can be set to 0 only by urfid."
>
> Since RTAS is entered using rfid, MSR[S] should remain whatever is the
> value in SRR1.
>
> And that's what POWER ISA is saying about the rfid instruction, in the
> synopsis of the instruction the bit 41 of the resulting MSR (aka MSR[S]) is
> not impacted.
>
> So there is no need to take care of this MSR bit in our code, but for sure,
> this should be well commented.
>
> Michael, do you agree?

Yep.

Can you send a v3 with updated change log and comment mentioning MSR_S
and MSR_LE, thanks.

cheers