Subject: Re: [PATCH V9 1/9] vhost: refine vhost and vringh kconfig

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 1 Apr 2020 11:57:14 -0400

On Wed, Apr 01, 2020 at 10:50:50PM +0800, Jason Wang wrote:
> 
> On 2020/4/1 下午10:27, Michael S. Tsirkin wrote:
> > On Wed, Apr 01, 2020 at 10:13:29PM +0800, Jason Wang wrote:
> > > On 2020/4/1 下午9:02, Christian Borntraeger wrote:
> > > > On 01.04.20 14:56, Christian Borntraeger wrote:
> > > > > On 01.04.20 14:50, Jason Wang wrote:
> > > > > > On 2020/4/1 下午7:21, Christian Borntraeger wrote:
> > > > > > > On 26.03.20 15:01, Jason Wang wrote:
> > > > > > > > Currently, CONFIG_VHOST depends on CONFIG_VIRTUALIZATION. But vhost is
> > > > > > > > not necessarily for VM since it's a generic userspace and kernel
> > > > > > > > communication protocol. Such dependency may prevent archs without
> > > > > > > > virtualization support from using vhost.
> > > > > > > > 
> > > > > > > > To solve this, a dedicated vhost menu is created under drivers so
> > > > > > > > CONIFG_VHOST can be decoupled out of CONFIG_VIRTUALIZATION.
> > > > > > > FWIW, this now results in vhost not being build with defconfig kernels (in todays
> > > > > > > linux-next).
> > > > > > > 
> > > > > > Hi Christian:
> > > > > > 
> > > > > > Did you meet it even with this commit https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=a4be40cbcedba9b5b714f3c95182e8a45176e42d?
> > > > > I simply used linux-next. The defconfig does NOT contain CONFIG_VHOST and therefore CONFIG_VHOST_NET and friends
> > > > > can not be selected.
> > > > > 
> > > > > $ git checkout next-20200401
> > > > > $ make defconfig
> > > > >     HOSTCC  scripts/basic/fixdep
> > > > >     HOSTCC  scripts/kconfig/conf.o
> > > > >     HOSTCC  scripts/kconfig/confdata.o
> > > > >     HOSTCC  scripts/kconfig/expr.o
> > > > >     LEX     scripts/kconfig/lexer.lex.c
> > > > >     YACC    scripts/kconfig/parser.tab.[ch]
> > > > >     HOSTCC  scripts/kconfig/lexer.lex.o
> > > > >     HOSTCC  scripts/kconfig/parser.tab.o
> > > > >     HOSTCC  scripts/kconfig/preprocess.o
> > > > >     HOSTCC  scripts/kconfig/symbol.o
> > > > >     HOSTCC  scripts/kconfig/util.o
> > > > >     HOSTLD  scripts/kconfig/conf
> > > > > *** Default configuration is based on 'x86_64_defconfig'
> > > > > #
> > > > > # configuration written to .config
> > > > > #
> > > > > 
> > > > > $ grep VHOST .config
> > > > > # CONFIG_VHOST is not set
> > > > > 
> > > > > > If yes, what's your build config looks like?
> > > > > > 
> > > > > > Thanks
> > > > This was x86. Not sure if that did work before.
> > > > On s390 this is definitely a regression as the defconfig files
> > > > for s390 do select VHOST_NET
> > > > 
> > > > grep VHOST arch/s390/configs/*
> > > > arch/s390/configs/debug_defconfig:CONFIG_VHOST_NET=m
> > > > arch/s390/configs/debug_defconfig:CONFIG_VHOST_VSOCK=m
> > > > arch/s390/configs/defconfig:CONFIG_VHOST_NET=m
> > > > arch/s390/configs/defconfig:CONFIG_VHOST_VSOCK=m
> > > > 
> > > > and this worked with 5.6, but does not work with next. Just adding
> > > > CONFIG_VHOST=m to the defconfig solves the issue, something like
> > > 
> > > Right, I think we probably need
> > > 
> > > 1) add CONFIG_VHOST=m to all defconfigs that enables
> > > CONFIG_VHOST_NET/VSOCK/SCSI.
> > > 
> > > or
> > > 
> > > 2) don't use menuconfig for CONFIG_VHOST, let NET/SCSI/VDPA just select it.
> > > 
> > > Thanks
> > OK I tried this:
> > 
> > diff --git a/drivers/vhost/Kconfig b/drivers/vhost/Kconfig
> > index 2523a1d4290a..a314b900d479 100644
> > --- a/drivers/vhost/Kconfig
> > +++ b/drivers/vhost/Kconfig
> > @@ -19,11 +19,10 @@ menuconfig VHOST
> >   	  This option is selected by any driver which needs to access
> >   	  the core of vhost.
> > -if VHOST
> > -
> >   config VHOST_NET
> >   	tristate "Host kernel accelerator for virtio net"
> >   	depends on NET && EVENTFD && (TUN || !TUN) && (TAP || !TAP)
> > +	select VHOST
> >   	---help---
> >   	  This kernel module can be loaded in host kernel to accelerate
> >   	  guest networking with virtio_net. Not to be confused with virtio_net
> > @@ -35,6 +34,7 @@ config VHOST_NET
> >   config VHOST_SCSI
> >   	tristate "VHOST_SCSI TCM fabric driver"
> >   	depends on TARGET_CORE && EVENTFD
> > +	select VHOST
> >   	default n
> >   	---help---
> >   	Say M here to enable the vhost_scsi TCM fabric module
> > @@ -44,6 +44,7 @@ config VHOST_VSOCK
> >   	tristate "vhost virtio-vsock driver"
> >   	depends on VSOCKETS && EVENTFD
> >   	select VIRTIO_VSOCKETS_COMMON
> > +	select VHOST
> >   	default n
> >   	---help---
> >   	This kernel module can be loaded in the host kernel to provide AF_VSOCK
> > @@ -57,6 +58,7 @@ config VHOST_VDPA
> >   	tristate "Vhost driver for vDPA-based backend"
> >   	depends on EVENTFD
> >   	select VDPA
> > +	select VHOST
> >   	help
> >   	  This kernel module can be loaded in host kernel to accelerate
> >   	  guest virtio devices with the vDPA-based backends.
> > @@ -78,5 +80,3 @@ config VHOST_CROSS_ENDIAN_LEGACY
> >   	  adds some overhead, it is disabled by default.
> >   	  If unsure, say "N".
> > -
> > -endif
> > 
> > 
> > But now CONFIG_VHOST is always "y", never "m".
> > Which I think will make it a built-in.
> > Didn't figure out why yet.
> 
> 
> Is it because the dependency of EVENTFD for CONFIG_VHOST?

Oh no, it's because I forgot to change menuconfig to config.

> Remove that one for this patch, I can get CONFIG_VHOST=m.
> 
> But according to documentation/kbuild/kconfig.rst, select is used for option
> without prompt.
> 
> Thanks
> 
> 
> >