Re: [RFC PATCH v2 01/20] migration/vmstate: Restrict vmstate_dummy to user-mode

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: "Philippe Mathieu-Daudé" <f4bug@amsat.org>
Cc: "Peter Maydell" <peter.maydell@linaro.org>,
	"Gerd Hoffmann" <kraxel@redhat.com>,
	"Daniel P. Berrangé" <berrange@redhat.com>,
	"Eduardo Habkost" <ehabkost@redhat.com>,
	"Juan Quintela" <quintela@redhat.com>,
	"Andrew Jeffery" <andrew@aj.id.au>,
	"Mark Cave-Ayland" <mark.cave-ayland@ilande.co.uk>,
	"Laurent Vivier" <laurent@vivier.eu>,
	"Andrew Baumann" <Andrew.Baumann@microsoft.com>,
	"QEMU Developers" <qemu-devel@nongnu.org>,
	qemu-arm <qemu-arm@nongnu.org>, "Joel Stanley" <joel@jms.id.au>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Subbaraya Sundeep" <sundeep.lkml@gmail.com>,
	"Artyom Tarasenko" <atar4qemu@gmail.com>,
	"Cédric Le Goater" <clg@kaod.org>
Subject: Re: [RFC PATCH v2 01/20] migration/vmstate: Restrict vmstate_dummy to user-mode
Date: Wed, 20 Jan 2021 11:03:14 +0000	[thread overview]
Message-ID: <20210120110314.GD2930@work-vm> (raw)
In-Reply-To: <004525a7-e8bb-9316-6ad5-ba5765471639@amsat.org>

* Philippe Mathieu-Daudé (f4bug@amsat.org) wrote:
> On 1/19/21 2:50 PM, Peter Maydell wrote:
> > On Sun, 17 Jan 2021 at 19:24, Philippe Mathieu-Daudé <f4bug@amsat.org> wrote:
> >>
> >> 'vmstate_dummy' is special and only used for user-mode. Rename
> >> it to something more specific.
> >> It was introduced restricted to user-mode in commit c71c3e99b8
> >> ("Add a vmstate_dummy struct for CONFIG_USER_ONLY") but this
> >> restriction was later removed in commit 6afc14e92ac ("migration:
> >> Fix warning caused by missing declaration of vmstate_dummy").
> >> Avoid the missing declaration warning by adding a stub for the
> >> symbol, and restore the #ifdef'ry.
> > 
> > So what is the actual use of vmstate_dummy ? I had a grep
> > through and as far as I can see the points where vmstate_cpu_common
> > is used are all in softmmu-only code.
> 
> No clue, maybe simply remnant from unfinished work?

Not sure either; but it looks like Paolo fixed some of it up in d9f24bf5724
a few months ago; prior to that cpu_exec_unrealizefn used it even on a
USER_ONLY build.

Dave

> > I tried this patch
> > and QEMU seems to compile OK:
> > 
> > diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
> > index 140fa32a5e3..a827417a4d8 100644
> > --- a/include/hw/core/cpu.h
> > +++ b/include/hw/core/cpu.h
> > @@ -1131,8 +1131,6 @@ bool target_words_bigendian(void);
> > 
> >  #ifdef CONFIG_SOFTMMU
> >  extern const VMStateDescription vmstate_cpu_common;
> > -#else
> > -#define vmstate_cpu_common vmstate_dummy
> >  #endif
> > 
> >  #define VMSTATE_CPU() {                                                     \
> 
> Great! Maybe even restricting VMSTATE_CPU() to softmmu-only:
> 
> -- >8 --
> @@ -1131,9 +1131,6 @@ bool target_words_bigendian(void);
> 
>  #ifdef CONFIG_SOFTMMU
>  extern const VMStateDescription vmstate_cpu_common;
> -#else
> -#define vmstate_cpu_common vmstate_dummy
> -#endif
> 
>  #define VMSTATE_CPU() {
>      \
>      .name = "parent_obj",
>      \
> @@ -1142,6 +1139,7 @@ extern const VMStateDescription vmstate_cpu_common;
>      .flags = VMS_STRUCT,
>      \
>      .offset = 0,
>      \
>  }
> +#endif
> 
>  #endif /* NEED_CPU_H */
> ---
> 
> I'll wait if David/Juan have any comment, else respin based
> on your patch.
> 
> Thanks,
> 
> Phil.
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK