On Wed, Jun 03, 2020 at 02:36:46PM +0800, Jason Wang wrote: > > On 2020/6/3 下午2:30, Michael S. Tsirkin wrote: > > On Wed, Jun 03, 2020 at 02:23:08PM +0800, Jason Wang wrote: > > > > BTW now I re-read it I don't understand __vhost_get_user_slow: > > > > > > > > > > > > static void __user *__vhost_get_user_slow(struct vhost_virtqueue *vq, > > > > void __user *addr, unsigned > > > > int size, > > > > int type) > > > > { > > > > int ret; > > > > > > > > ret = translate_desc(vq, (u64)(uintptr_t)addr, size, > > > > vq->iotlb_iov, > > > > ARRAY_SIZE(vq->iotlb_iov), > > > > VHOST_ACCESS_RO); > > > > > > > > .. > > > > } > > > > > > > > how does this work? how can we cast a pointer to guest address without > > > > adding any offsets? > > > > > > I'm not sure I get you here. What kind of offset did you mean? > > > > > > Thanks > > OK so points: > > > > 1. type argument seems unused. Right? > > > Yes, we can remove that. > > > > 2. Second argument to translate_desc is a GPA, isn't it? > > > No, it's IOVA, this function will be called only when IOTLB is enabled. > > Thanks
Right IOVA. Point stands how does it make sense to cast a userspace pointer to an IOVA? I guess it's just because it's talking to qemu actually, so it's abusing the notation a bit ... > > > Here we cast a userspace address to this type. What if it > > matches a valid GPA by mistake? > >