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Date:   Wed, 5 Aug 2020 07:45:46 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [PATCH v2 03/24] virtio: allow __virtioXX, __leXX in config space
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On Wed, Aug 05, 2020 at 02:28:23PM +0800, Jason Wang wrote:
> 
> On 2020/8/4 上午4:58, Michael S. Tsirkin wrote:
> > Currently all config space fields are of the type __uXX.
> > This confuses people and some drivers (notably vdpa)
> > access them using CPU endian-ness - which only
> > works well for legacy or LE platforms.
> > 
> > Update virtio_cread/virtio_cwrite macros to allow __virtioXX
> > and __leXX field types. Follow-up patches will convert
> > config space to use these types.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > ---
> >   include/linux/virtio_config.h | 50 +++++++++++++++++++++++++++++++++--
> >   1 file changed, 48 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/virtio_config.h b/include/linux/virtio_config.h
> > index 3b4eae5ac5e3..64da491936f7 100644
> > --- a/include/linux/virtio_config.h
> > +++ b/include/linux/virtio_config.h
> > @@ -6,6 +6,7 @@
> >   #include <linux/bug.h>
> >   #include <linux/virtio.h>
> >   #include <linux/virtio_byteorder.h>
> > +#include <linux/compiler_types.h>
> >   #include <uapi/linux/virtio_config.h>
> >   struct irq_affinity;
> > @@ -287,12 +288,57 @@ static inline __virtio64 cpu_to_virtio64(struct virtio_device *vdev, u64 val)
> >   	return __cpu_to_virtio64(virtio_is_little_endian(vdev), val);
> >   }
> > +/*
> > + * Only the checker differentiates between __virtioXX and __uXX types. But we
> > + * try to share as much code as we can with the regular GCC build.
> > + */
> > +#if !defined(CONFIG_CC_IS_GCC) && !defined(__CHECKER__)
> > +
> > +/* Not a checker - we can keep things simple */
> > +#define __virtio_native_typeof(x) typeof(x)
> > +
> > +#else
> > +
> > +/*
> > + * We build this out of a couple of helper macros in a vain attempt to
> > + * help you keep your lunch down while reading it.
> > + */
> > +#define __virtio_pick_value(x, type, then, otherwise)			\
> > +	__builtin_choose_expr(__same_type(x, type), then, otherwise)
> > +
> > +#define __virtio_pick_type(x, type, then, otherwise)			\
> > +	__virtio_pick_value(x, type, (then)0, otherwise)
> > +
> > +#define __virtio_pick_endian(x, x16, x32, x64, otherwise)			\
> > +	__virtio_pick_type(x, x16, __u16,					\
> > +		__virtio_pick_type(x, x32, __u32,				\
> > +			__virtio_pick_type(x, x64, __u64,			\
> > +				otherwise)))
> > +
> > +#define __virtio_native_typeof(x) typeof(					\
> > +	__virtio_pick_type(x, __u8, __u8,					\
> > +		__virtio_pick_endian(x, __virtio16, __virtio32, __virtio64,	\
> > +			__virtio_pick_endian(x, __le16, __le32, __le64,		\
> > +				__virtio_pick_endian(x, __u16, __u32, __u64,	\
> > +					/* No other type allowed */		\
> > +					(void)0)))))
> > +
> > +#endif
> > +
> > +#define __virtio_native_type(structname, member) \
> > +	__virtio_native_typeof(((structname*)0)->member)
> > +
> > +#define __virtio_typecheck(structname, member, val) \
> > +		/* Must match the member's type, and be integer */ \
> > +		typecheck(__virtio_native_type(structname, member), (val))
> > +
> > +
> >   /* Config space accessors. */
> >   #define virtio_cread(vdev, structname, member, ptr)			\
> >   	do {								\
> >   		might_sleep();						\
> >   		/* Must match the member's type, and be integer */	\
> > -		if (!typecheck(typeof((((structname*)0)->member)), *(ptr))) \
> > +		if (!__virtio_typecheck(structname, member, *(ptr)))	\
> >   			(*ptr) = 1;					\
> 
> 
> A silly question,  compare to using set()/get() directly, what's the value
> of the accessors macro here?
> 
> Thanks

get/set don't convert to the native endian, I guess that's why
drivers use cread/cwrite. It is also nice that there's type
safety, checking the correct integer width is used.

> 
> >   									\
> >   		switch (sizeof(*ptr)) {					\
> > @@ -322,7 +368,7 @@ static inline __virtio64 cpu_to_virtio64(struct virtio_device *vdev, u64 val)
> >   	do {								\
> >   		might_sleep();						\
> >   		/* Must match the member's type, and be integer */	\
> > -		if (!typecheck(typeof((((structname*)0)->member)), *(ptr))) \
> > +		if (!__virtio_typecheck(structname, member, *(ptr)))	\
> >   			BUG_ON((*ptr) == 1);				\
> >   									\
> >   		switch (sizeof(*ptr)) {					\