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Re: [PATCH 2/2] meson: Warn when TCI is selected but TCG backend is avai

From: Stefan Weil
Subject: Re: [PATCH 2/2] meson: Warn when TCI is selected but TCG backend is available
Date: Fri, 22 Jan 2021 10:56:25 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.16; rv:78.0) Gecko/20100101 Thunderbird/78.6.1

Am 22.01.21 um 10:47 schrieb Thomas Huth:

On 22/01/2021 10.46, Philippe Mathieu-Daudé wrote:
On Fri, Jan 22, 2021 at 10:43 AM Thomas Huth <thuth@redhat.com> wrote:
On 22/01/2021 10.22, Philippe Mathieu-Daudé wrote:
Some new users get confused with 'TCG' and 'TCI', and enable TCI
support expecting to enable TCG.

Emit a warning when native TCG backend is available on the
host architecture, mentioning this is a suboptimal configuration.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
   meson.build | 3 +++
   1 file changed, 3 insertions(+)

diff --git a/meson.build b/meson.build
index 0a645e54662..7aa52d306c6 100644
--- a/meson.build
+++ b/meson.build
@@ -234,6 +234,9 @@
         error('Unsupported CPU @0@, try --enable-tcg-interpreter'.format(cpu))
+  if 'CONFIG_TCG_INTERPRETER' in config_host and cpu in supported_cpus +    warning('Experimental TCI requested while native TCG is available on @0@, suboptimal performance expected'.format(cpu))
+  endif
     accelerators += 'CONFIG_TCG'
     config_host += { 'CONFIG_TCG': 'y' }

Reviewed-by: Thomas Huth <thuth@redhat.com>

... maybe you could also add some wording to the help text of the configure
script? Or maybe we could simply rename the "--enable-tcg-interpreter"
option into "--enable-tci" to avoid confusion with the normal TCG ?

I also thought about renaming as --enable-experimental-tci but then doubt that
would help, some users just want to enable everything :)

Then we should rename it into --disable-native-tcg ;-)


Both patches are fine (also optionally with the suggested addition of "slow"), so

Reviewed-by: Stefan Weil <sw@weilnetz.de>

I think that --enable-tci would increase the TCI/TCG confusion and suggest to keep the current --enable-tcg-interpreter as most experts know that an interpreter is usually something which is slow.

As soon as time permits I also plan to make a new effort to integrate TCI as a run time option. Some years ago it was not possible to have code which supports both native and interpreted TCG, but this might have changed now. If it is possible, this would simplify CI a lot, and users could select the interpreter via a command line argument.


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