Re: [Qemu-devel] [PATCHv2 3/4] cpuid: disable pv eoi for 1.1 and older compat types

[Michael S. Tsirkin]

On Mon, Aug 27, 2012 at 07:12:27PM +0000, Blue Swirl wrote:
> On Mon, Aug 27, 2012 at 7:06 PM, Michael S. Tsirkin <address@hidden> wrote:
> > On Mon, Aug 27, 2012 at 06:58:29PM +0000, Blue Swirl wrote:
> >> On Mon, Aug 27, 2012 at 12:20 PM, Michael S. Tsirkin <address@hidden> 
> >> wrote:
> >> > In preparation for adding PV EOI support, disable PV EOI by default for
> >> > 1.1 and older machine types, to avoid CPUID changing during migration.
> >> >
> >> > PV EOI can still be enabled/disabled by specifying it explicitly.
> >> >     Enable for 1.1
> >> >     -M pc-1.1 -cpu kvm64,+kvm_pv_eoi
> >> >     Disable for 1.2
> >> >     -M pc-1.2 -cpu kvm64,-kvm_pv_eoi
> >> >
> >> > Signed-off-by: Michael S. Tsirkin <address@hidden>
> >> > ---
> >> >  hw/Makefile.objs  |  2 +-
> >> >  hw/cpu_flags.c    | 32 ++++++++++++++++++++++++++++++++
> >> >  hw/cpu_flags.h    |  9 +++++++++
> >> >  hw/pc_piix.c      |  2 ++
> >> >  target-i386/cpu.c |  8 ++++++++
> >> >  5 files changed, 52 insertions(+), 1 deletion(-)
> >> >  create mode 100644 hw/cpu_flags.c
> >> >  create mode 100644 hw/cpu_flags.h
> >> >
> >> > diff --git a/hw/Makefile.objs b/hw/Makefile.objs
> >> > index 850b87b..3f2532a 100644
> >> > --- a/hw/Makefile.objs
> >> > +++ b/hw/Makefile.objs
> >> > @@ -1,5 +1,5 @@
> >> >  hw-obj-y = usb/ ide/
> >> > -hw-obj-y += loader.o
> >> > +hw-obj-y += loader.o cpu_flags.o
> >> >  hw-obj-$(CONFIG_VIRTIO) += virtio-console.o
> >> >  hw-obj-$(CONFIG_VIRTIO_PCI) += virtio-pci.o
> >> >  hw-obj-y += fw_cfg.o
> >> > diff --git a/hw/cpu_flags.c b/hw/cpu_flags.c
> >> > new file mode 100644
> >> > index 0000000..2422d20
> >> > --- /dev/null
> >> > +++ b/hw/cpu_flags.c
> >> > @@ -0,0 +1,32 @@
> >> > +/*
> >> > + * CPU compatibility flags.
> >> > + *
> >> > + * Copyright (c) 2012 Red Hat Inc.
> >> > + * Author: Michael S. Tsirkin.
> >> > + *
> >> > + * This program is free software; you can redistribute it and/or modify
> >> > + * it under the terms of the GNU General Public License as published by
> >> > + * the Free Software Foundation; either version 2 of the License, or
> >> > + * (at your option) any later version.
> >> > + *
> >> > + * This program is distributed in the hope that it will be useful,
> >> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> >> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> >> > + * GNU General Public License for more details.
> >> > + *
> >> > + * You should have received a copy of the GNU General Public License 
> >> > along
> >> > + * with this program; if not, see <http://www.gnu.org/licenses/>.
> >> > + */
> >> > +#include "hw/cpu_flags.h"
> >> > +
> >> > +static bool __kvm_pv_eoi_disabled;
> >>
> >> Don't use identifiers with leading underscores.
> >
> > C99 spec says "
> > Any other predefined macro names
> > shall begin with a leading underscore followed by an uppercase letter or
> > a second underscore.
> > "
> >
> > what are chances of compiler predefining macro __kvm_pv_eoi_disabled?
> 
> Why do you even consider that since it's trivially easy to use
> something else? If a standard (and HACKING in our case) specifies
> something, why do you want to fight it?

I missed this in HACKING, you are right:

        2.4. Reserved namespaces in C and POSIX
        Underscore capital, double underscore, and underscore 't' suffixes
        should be avoided.

so _kvm_pv_eoi_disabled is ok __kvm_pv_eoi_disabled is not.
Will fix.

> >
> > But OK, will rename _kvm_pv_eoi_disabled.
> > _ + lower case is guaranteed OK.
> 
> No, just use kvm_pv_eoi_disabled, the underscore is useless.

It isn't useless, this avoids conflict with function name.
_ says it's an internal variable used to implement kvm_pv_eoi_disabled
in a very clear way.

-- 
MST