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Date: Wed, 14 Aug 2019 13:34:32 -0500
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Subject: Re: [PATCH v5 2/7] powerpc/kernel: Add ucall_norets() ultravisor call
 handler
Message-ID: <20190814183432.GG31406@gate.crashing.org>
References: <20190808040555.2371-1-cclaudio@linux.ibm.com>
 <20190808040555.2371-3-cclaudio@linux.ibm.com>
 <87wofgqb2g.fsf@concordia.ellerman.id.au>
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Cc: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>,
 Michael Anderson <andmike@linux.ibm.com>, Ram Pai <linuxram@us.ibm.com>,
 Claudio Carvalho <cclaudio@linux.ibm.com>, kvm-ppc@vger.kernel.org,
 Bharata B Rao <bharata@linux.ibm.com>, linuxppc-dev@ozlabs.org,
 Ryan Grimm <grimm@linux.ibm.com>,
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 Guerney Hunt <gdhh@linux.ibm.com>, Thiago Bauermann <bauerman@linux.ibm.com>
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On Wed, Aug 14, 2019 at 08:46:15PM +1000, Michael Ellerman wrote:
> Claudio Carvalho <cclaudio@linux.ibm.com> writes:
> > +_GLOBAL(ucall_norets)
> > +EXPORT_SYMBOL_GPL(ucall_norets)
> > +	mfcr	r0
> > +	stw	r0,8(r1)
> > +
> > +	sc	2		/* Invoke the ultravisor */
> > +
> > +	lwz	r0,8(r1)
> > +	mtcrf	0xff,r0
> > +	blr			/* Return r3 = status */
> 
> Paulus points that we shouldn't need to save CR here. Our caller will
> have already saved it if it needed to, and we don't use CR in this
> function so we don't need to save it.
> 
> That's assuming the Ultravisor follows the hcall ABI in which CR2-4 are
> non-volatile (PAPR § 14.5.3).

And assuming the ultravisor already clears (or sets, or whatever) all CR
fields it does not want to leak the contents of (which it also should,
of course).

> I know plpar_hcall_norets() does save CR, but it shouldn't need to, that
> seems to be historical. aka. no one knows why it does it but it always
> has.


Segher

From mboxrd@z Thu Jan  1 00:00:00 1970
From: Segher Boessenkool <segher@kernel.crashing.org>
Date: Wed, 14 Aug 2019 18:34:32 +0000
Subject: Re: [PATCH v5 2/7] powerpc/kernel: Add ucall_norets() ultravisor call handler
Message-Id: <20190814183432.GG31406@gate.crashing.org>
List-Id: <kvm-ppc.vger.kernel.org>
References: <20190808040555.2371-1-cclaudio@linux.ibm.com>
 <20190808040555.2371-3-cclaudio@linux.ibm.com>
 <87wofgqb2g.fsf@concordia.ellerman.id.au>
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To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>, Michael Anderson <andmike@linux.ibm.com>, Ram Pai <linuxram@us.ibm.com>, Claudio Carvalho <cclaudio@linux.ibm.com>, kvm-ppc@vger.kernel.org, Bharata B Rao <bharata@linux.ibm.com>, linuxppc-dev@ozlabs.org, Ryan Grimm <grimm@linux.ibm.com>, Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>, Guerney Hunt <gdhh@linux.ibm.com>, Thiago Bauermann <bauerman@linux.ibm.com>

On Wed, Aug 14, 2019 at 08:46:15PM +1000, Michael Ellerman wrote:
> Claudio Carvalho <cclaudio@linux.ibm.com> writes:
> > +_GLOBAL(ucall_norets)
> > +EXPORT_SYMBOL_GPL(ucall_norets)
> > +	mfcr	r0
> > +	stw	r0,8(r1)
> > +
> > +	sc	2		/* Invoke the ultravisor */
> > +
> > +	lwz	r0,8(r1)
> > +	mtcrf	0xff,r0
> > +	blr			/* Return r3 =3D status */
>=20
> Paulus points that we shouldn't need to save CR here. Our caller will
> have already saved it if it needed to, and we don't use CR in this
> function so we don't need to save it.
>=20
> That's assuming the Ultravisor follows the hcall ABI in which CR2-4 are
> non-volatile (PAPR =A7 14.5.3).

And assuming the ultravisor already clears (or sets, or whatever) all CR
fields it does not want to leak the contents of (which it also should,
of course).

> I know plpar_hcall_norets() does save CR, but it shouldn't need to, that
> seems to be historical. aka. no one knows why it does it but it always
> has.


Segher