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

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Date:   Tue, 03 May 2022 13:16:29 -0300
From:   Fabiano Rosas <farosas@...ux.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Laurent Dufour <ldufour@...ux.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v2] powerpc/rtas: Keep MSR[RI] set when calling RTAS

Michael Ellerman <mpe@...erman.id.au> 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.