Re: [PATCH kvmtool 1/1] linux/virtio_pci.h: Include kernel.h to provide the __KERNEL_DIV_ROUND_UP define

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Mar 13, 2026 at 12:53:06PM +0000, Will Deacon wrote:
> [+mst]
> 
> On Fri, Feb 27, 2026 at 08:42:24AM +0800, hbuxiaofei wrote:
> > GCC Version:
> >   gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-44)
> > 
> > CC       builtin-balloon.o
> > In file included from include/kvm/pci.h:7:0,
> >                  from include/kvm/vfio.h:6,
> >                  from include/kvm/kvm-config.h:5,
> >                  from include/kvm/kvm.h:6,
> >                  from builtin-balloon.c:9:
> > include/linux/virtio_pci.h:326:2: error: implicit declaration of function ‘__KERNEL_DIV_ROUND_UP’ [-Werror=implicit-function-declaration]
> >   __le64 supported_caps[MAX_CAP_ID];
> >   ^
> > include/linux/virtio_pci.h:326:9: error: variably modified ‘supported_caps’ at file scope
> >   __le64 supported_caps[MAX_CAP_ID];
> >          ^
> > cc1: all warnings being treated as errors
> > make: *** [builtin-balloon.o] Error 1
> > 
> > Signed-off-by: hbuxiaofei <hbuxiaofei@gmail.com>
> > ---
> >  include/linux/virtio_pci.h | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/include/linux/virtio_pci.h b/include/linux/virtio_pci.h
> > index e732e34..fb2f66a 100644
> > --- a/include/linux/virtio_pci.h
> > +++ b/include/linux/virtio_pci.h
> > @@ -39,6 +39,7 @@
> >  #ifndef _LINUX_VIRTIO_PCI_H
> >  #define _LINUX_VIRTIO_PCI_H
> >  
> > +#include <linux/kernel.h>
> >  #include <linux/types.h>
> >  #include <linux/const.h>
> 
> Hmm. This header is imported directly from the kernel sources, so it
> doesn't make sense to patch it by hand as changes will be overwritten
> next time we run util/update_headers.sh.
> 
> Looking at the kernel history, 205dd7a5d6ad ("virtio_pci: drop kernel.h")
> suggests that linux/const.h is sufficient for __KERNEL_DIV_ROUND_UP, so
> we probably just want to copy that header over (e.g. like the diff below).
> 
> Will
> 

Exactly.

> --->8
> 
> diff --git a/include/linux/const.h b/include/linux/const.h
> new file mode 100644
> index 0000000..b8f629e
> --- /dev/null
> +++ b/include/linux/const.h
> @@ -0,0 +1,53 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +/* const.h: Macros for dealing with constants.  */
> +
> +#ifndef _UAPI_LINUX_CONST_H
> +#define _UAPI_LINUX_CONST_H
> +
> +/* Some constant macros are used in both assembler and
> + * C code.  Therefore we cannot annotate them always with
> + * 'UL' and other type specifiers unilaterally.  We
> + * use the following macros to deal with this.
> + *
> + * Similarly, _AT() will cast an expression with a type in C, but
> + * leave it unchanged in asm.
> + */
> +
> +#ifdef __ASSEMBLY__
> +#define _AC(X,Y)	X
> +#define _AT(T,X)	X
> +#else
> +#define __AC(X,Y)	(X##Y)
> +#define _AC(X,Y)	__AC(X,Y)
> +#define _AT(T,X)	((T)(X))
> +#endif
> +
> +#define _UL(x)		(_AC(x, UL))
> +#define _ULL(x)		(_AC(x, ULL))
> +
> +#define _BITUL(x)	(_UL(1) << (x))
> +#define _BITULL(x)	(_ULL(1) << (x))
> +
> +#if !defined(__ASSEMBLY__)
> +/*
> + * Missing asm support
> + *
> + * __BIT128() would not work in the asm code, as it shifts an
> + * 'unsigned __int128' data type as direct representation of
> + * 128 bit constants is not supported in the gcc compiler, as
> + * they get silently truncated.
> + *
> + * TODO: Please revisit this implementation when gcc compiler
> + * starts representing 128 bit constants directly like long
> + * and unsigned long etc. Subsequently drop the comment for
> + * GENMASK_U128() which would then start supporting asm code.
> + */
> +#define _BIT128(x)	((unsigned __int128)(1) << (x))
> +#endif
> +
> +#define __ALIGN_KERNEL(x, a)		__ALIGN_KERNEL_MASK(x, (__typeof__(x))(a) - 1)
> +#define __ALIGN_KERNEL_MASK(x, mask)	(((x) + (mask)) & ~(mask))
> +
> +#define __KERNEL_DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
> +
> +#endif /* _UAPI_LINUX_CONST_H */
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index df42d63..6c22f1c 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -8,9 +8,6 @@
>  #define round_down(x, y)	((x) & ~__round_mask(x, y))
>  
>  #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
> -#ifndef __KERNEL_DIV_ROUND_UP
> -#define __KERNEL_DIV_ROUND_UP(n,d)	DIV_ROUND_UP(n,d)
> -#endif
>  
>  #define ALIGN(x,a)		__ALIGN_MASK(x,(typeof(x))(a)-1)
>  #define __ALIGN_MASK(x,mask)	(((x)+(mask))&~(mask))