Re: [Xen-devel] [PATCH] xen/common: Drop function calls for Xen compile/version information

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Jan 18, 2017 at 04:21:06PM +0000, Roger Pau Monné wrote:
> On Wed, Jan 18, 2017 at 11:10:40AM -0500, Konrad Rzeszutek Wilk wrote:
> > On Wed, Jan 18, 2017 at 10:13:34AM +0000, Roger Pau Monné wrote:
> > > On Tue, Jan 17, 2017 at 03:46:30PM -0500, Konrad Rzeszutek Wilk wrote:
> > > > > > Is this patch of yours that neccessary? Could at least some of the
> > > > > > functions still exist?
> > > > > 
> > > > > Well.  This patch is manually doing what LTO would do automatically 
> > > > > when
> > > > > it has a cross-translation-unit view of things, and come to the
> > > > > conclusion that in all cases, inlining cross-unit will make the 
> > > > > calling
> > > > > code shorter, and allow the functions to be elided entirely.
> > > > > 
> > > > > I take it from this that noone has tried livepatching an LTO build of 
> > > > > Xen.
> > > > 
> > > > Roger did build it (FreeBSD uses it), but I don't think he tried
> > > > the test-cases.
> > > 
> > > I've tried livepatch, but not LTO, neither on it's own.
> > 
> > Gotcha.
> > > 
> > > > Wait, I think he may have as his patch: 
> > > > f8c66c2ad2efdb281e4ebf15bf329d73c4f02ce7
> > > > Author: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> > > > Date:   Tue May 3 12:55:09 2016 +0200
> > > > 
> > > >     xen/xsplice: add ELFOSABI_FREEBSD as a supported OSABI for payloads
> > > > 
> > > > implies that he did build an payload (and I remember us trying to figure
> > > > out if the issues he had been hitting were due to LTO or something 
> > > > else).
> > > 
> > > IIRC issues where related to not translating error codes from the return 
> > > of the
> > > livepatch hypercalls, and the patch upload tool refusing to upload 
> > > patches with
> > > ELFOSABI_FREEBSD instead of ELFOSABI_NONE (which I assume is what Linux 
> > > uses).
> > > 
> > > It's been sometime since I've tried livepatch with FreeBSD, so I could 
> > > give it
> > > a spin if you think there's a chance things might have broken.
> > 
> > That is OK. 
> > 
> > I think I can also try myself to build Xen with LTO on Linux and see how
> > the test-cases fare.
> > 
> > Not sure what is involved but I will find out.
> > 
> > And to answer Andrew's question - No, nobody tried LTO with livepatching 
> > then :-(
> 
> TBH, I think nobody has tried LTO on it's own in a long time, much less with
> livepatch. Adding Wei who recently sent some LTO makefile fixes, maybe he 
> tried
> it.
> 

LTO is now controlled by CONFIG_LTO in kconfig.

Xen build system is broken when LTO is enabled. Furthermore GCC
toolchain can't handle LTO yet. 

Konrad, feel free to fix LTO build. :-)

Wei.

> Roger.

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