Re: [PATCH v2] tools: probe for existence of qemu-xen stderr trace backend.

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Ian Campbell [mailto:ian.campbell@citrix.com]
> Sent: 11 February 2016 12:11
> To: Ian Jackson; Wei Liu; xen-devel@lists.xen.org; Stefano Stabellini
> Cc: Ian Campbell; Paul Durrant; Anthony Perard
> Subject: [PATCH v2] tools: probe for existence of qemu-xen stderr trace
> backend.
> 
> QEMU upstream commit ed7f5f1d8db0 ("trace: convert stderr backend to
> log") renamed the "stderr" trace backend to "log", which breaks the
> xen build when pointed at a QEMU tree after that point:
> 
> ./configure of QEMU fail with:
> "ERROR: invalid trace backends
>         Please choose supported trace backends."
> 
> Upstream also changed, in baf86d6b3ca0 ("trace: switch default backend
> to "log""), to use "log" as the default backend (previously it was
> "nop").
> 
> Use ./scripts/tracetool.py to check for the presence of the stderr
> backend and if it is present then explicitly enable it. If the stderr
> backend is not present then assume a newer QEMU which defaults to
> "log" and simply accept that default (there is a 1 commit window
> upstream where this would result in no trace backend being enabled).
> 
> The check is done using the older (deprecated?) --check-backend/--backend
> variant of the tracetool.py options rather than the new plural
> versions since the singular was supported even by very old versions of
> QEMU.  New QEMU has compatibility code but if/when that is removed we
> will still do the right thing i.e. no explict configuiration resulting
> in the upstream default (currently "log").
> 
> If the explicit selection of the "stderr" backend is required then it
> is now done unconditionally (not depending on debug=y), which is
> simpler to arrange here but also matches the newer upstream's default
> to "log" which is not conditional on debug being enabled either.
> 
> Tested with current qemu-xen-unstable (e9d8252) and current QEMU
> upstream master (88c73d1), both out of tree via
> QEMU_UPSTREAM_URL=/path/to/qemu-xen.git.
> 
> Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
> Cc: Paul Durrant <paul.durrant@citrix.com>

LGTM.

Reviewed-by: Paul Durrant <paul.durrant@citrix.com>

> Cc: Anthony PERARD <anthony.perard@citrix.com>
> ---
> v2: Just probe for stderr, otherwise assume the new upstream default
>     of log is what we will get anyway. Substantially rewrite commit
>     message.
> ---
>  tools/Makefile | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/Makefile b/tools/Makefile
> index cb8652c..3f45fb9 100644
> --- a/tools/Makefile
> +++ b/tools/Makefile
> @@ -228,7 +228,7 @@ qemu-xen-dir-force-update: qemu-xen-dir-find
>  	fi
> 
>  ifeq ($(debug),y)
> -QEMU_XEN_ENABLE_DEBUG := --enable-debug --enable-trace-
> backend=stderr
> +QEMU_XEN_ENABLE_DEBUG := --enable-debug
>  else
>  QEMU_XEN_ENABLE_DEBUG :=
>  endif
> @@ -240,8 +240,14 @@ subdir-all-qemu-xen-dir: qemu-xen-dir-find
>  		source=.; \
>  	fi; \
>  	cd qemu-xen-dir; \
> +	if $$source/scripts/tracetool.py --check-backend --backend stderr ;
> then \
> +		enable_trace_backend='--enable-trace-backend=stderr'; \
> +	else \
> +		enable_trace_backend='' ; \
> +	fi ; \
>  	$$source/configure --enable-xen --target-list=i386-softmmu \
>  		$(QEMU_XEN_ENABLE_DEBUG) \
> +		$$enable_trace_backend \
>  		--prefix=$(LIBEXEC) \
>  		--libdir=$(LIBEXEC_LIB) \
>  		--includedir=$(LIBEXEC_INC) \
> --
> 2.1.4