Re: Possible regression by ab037dd87a2f (powerpc/vdso: Switch VDSO to generic C implementation.)

From: Segher Boessenkool <segher@kernel.crashing.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: parkerderek86@gmail.com, pmenzel@molgen.mpg.de,
	laboger@linux.vnet.ibm.com, xaionaro@gmail.com,
	Paul Murphy <murp@ibm.com>,
	paulus@samba.org, murphyp@linux.ibm.com,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: Possible regression by ab037dd87a2f (powerpc/vdso: Switch VDSO to generic C implementation.)
Date: Mon, 2 Aug 2021 13:07:04 -0500	[thread overview]
Message-ID: <20210802180704.GO1583@gate.crashing.org> (raw)
In-Reply-To: <87czqwl67h.fsf@mpe.ellerman.id.au>

On Mon, Aug 02, 2021 at 04:18:58PM +1000, Michael Ellerman wrote:
> > Go up to this point has only used the vdso function __kernel_clock_gettime; it
> > is the only entry point which would need to explicitly avoid R30 for
> > Go's sake.
> 
> I thought about limiting the workaround to just that code, but it seemed
> silly and likely to come back to bite us. Once the compilers decides to
> spill a non-volatile there are plenty of other ones to choose from.

It can be cheaper to spill N..31 consecutively, using stmw for example.
For 64-bit Power implementations it doesn't currently make any
difference.  Since none of this will be inlined it doesn't have any real
impact.

(This also happens with -m32 -fpic, which always sets GPR30 as fixed, it
is the offset table register.  With those flags -ffixed-r30 doesn't do
anything btw (r30 already *is* a fixed function register), and this will
not work with a Go that clobbers r30.  But this is academic :-) )

Segher