From: Michael Ellerman <mpe@ellerman•id.au>
To: Laurent Dufour <ldufour@linux•ibm.com>,
Fabiano Rosas <farosas@linux•ibm.com>
Cc: linuxppc-dev@lists•ozlabs.org, linux-kernel@vger•kernel.org,
stable@vger•kernel.org, Nicholas Piggin <npiggin@gmail•com>
Subject: Re: [PATCH v2] powerpc/rtas: Keep MSR[RI] set when calling RTAS
Date: Wed, 04 May 2022 14:27:38 +1000 [thread overview]
Message-ID: <87h765vs5h.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <60195753-93fc-ced7-c250-da65c05508af@linux.ibm.com>
Laurent Dufour <ldufour@linux•ibm.com> writes:
> On 03/05/2022, 18:16:29, Fabiano Rosas wrote:
>> Michael Ellerman <mpe@ellerman•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.
>
> 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
next prev parent reply other threads:[~2022-05-04 4:28 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-01 14:06 [PATCH v2] powerpc/rtas: Keep MSR[RI] set when calling RTAS Laurent Dufour
2022-04-13 5:58 ` Nicholas Piggin
2022-04-21 14:09 ` Laurent Dufour
2022-05-03 15:06 ` Michael Ellerman
2022-05-03 16:16 ` Fabiano Rosas
2022-05-03 16:47 ` Laurent Dufour
2022-05-04 4:27 ` Michael Ellerman [this message]
2022-05-04 4:26 ` Michael Ellerman
2022-05-03 16:23 ` Laurent Dufour
2022-05-04 5:59 ` Michael Ellerman
2022-05-04 11:01 ` Laurent Dufour
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87h765vs5h.fsf@mpe.ellerman.id.au \
--to=mpe@ellerman$(echo .)id.au \
--cc=farosas@linux$(echo .)ibm.com \
--cc=ldufour@linux$(echo .)ibm.com \
--cc=linux-kernel@vger$(echo .)kernel.org \
--cc=linuxppc-dev@lists$(echo .)ozlabs.org \
--cc=npiggin@gmail$(echo .)com \
--cc=stable@vger$(echo .)kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox