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Date:   Wed, 11 Jan 2023 12:26:38 +0900
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Subject: Re: [RFC PATCH 2/9] vringh: remove vringh_iov and unite to
 vringh_kiov
To:     Jason Wang <jasowang@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Rusty Russell <rusty@rustcorp.com.au>, kvm@vger.kernel.org,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20221227022528.609839-1-mie@igel.co.jp>
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From:   Shunsuke Mie <mie@igel.co.jp>
In-Reply-To: <CACGkMEvmZ5MEX4WMa3JhzT404C2uhsNk0nnkYBRtvLPhNTSzHQ@mail.gmail.com>
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On 2022/12/28 15:36, Jason Wang wrote:
> On Tue, Dec 27, 2022 at 3:06 PM Shunsuke Mie <mie@igel.co.jp> wrote:
>> 2022年12月27日(火) 15:04 Jason Wang <jasowang@redhat.com>:
>>> On Tue, Dec 27, 2022 at 10:25 AM Shunsuke Mie <mie@igel.co.jp> wrote:
>>>> struct vringh_iov is defined to hold userland addresses. However, to use
>>>> common function, __vring_iov, finally the vringh_iov converts to the
>>>> vringh_kiov with simple cast. It includes compile time check code to make
>>>> sure it can be cast correctly.
>>>>
>>>> To simplify the code, this patch removes the struct vringh_iov and unifies
>>>> APIs to struct vringh_kiov.
>>>>
>>>> Signed-off-by: Shunsuke Mie <mie@igel.co.jp>
>>> While at this, I wonder if we need to go further, that is, switch to
>>> using an iov iterator instead of a vringh customized one.
>> I didn't see the iov iterator yet, thank you for informing me.
>> Is that iov_iter? https://lwn.net/Articles/625077/
> Exactly.

I've investigated the iov_iter, vhost and related APIs. As a result, I
think that it is not easy to switch to use the iov_iter. Because, the
design of vhost and vringh is different.

The iov_iter has vring desc info and meta data of transfer method. The
vhost provides generic transfer function for the iov_iter. In constrast,
vringh_iov just has vring desc info. The vringh provides transfer functions
for each methods.

In the future, it is better to use common data structure and APIs between
vhost and vringh (or merge completely), but it requires a lot of 
changes, so I'd like to just
organize data structure in vringh as a first step in this patch.


Best

> Thanks
>
>>> Thanks
>>>
>>>> ---
>>>>   drivers/vhost/vringh.c | 32 ++++++------------------------
>>>>   include/linux/vringh.h | 45 ++++--------------------------------------
>>>>   2 files changed, 10 insertions(+), 67 deletions(-)
>>>>
>>>> diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
>>>> index 828c29306565..aa3cd27d2384 100644
>>>> --- a/drivers/vhost/vringh.c
>>>> +++ b/drivers/vhost/vringh.c
>>>> @@ -691,8 +691,8 @@ EXPORT_SYMBOL(vringh_init_user);
>>>>    * calling vringh_iov_cleanup() to release the memory, even on error!
>>>>    */
>>>>   int vringh_getdesc_user(struct vringh *vrh,
>>>> -                       struct vringh_iov *riov,
>>>> -                       struct vringh_iov *wiov,
>>>> +                       struct vringh_kiov *riov,
>>>> +                       struct vringh_kiov *wiov,
>>>>                          bool (*getrange)(struct vringh *vrh,
>>>>                                           u64 addr, struct vringh_range *r),
>>>>                          u16 *head)
>>>> @@ -708,26 +708,6 @@ int vringh_getdesc_user(struct vringh *vrh,
>>>>          if (err == vrh->vring.num)
>>>>                  return 0;
>>>>
>>>> -       /* We need the layouts to be the identical for this to work */
>>>> -       BUILD_BUG_ON(sizeof(struct vringh_kiov) != sizeof(struct vringh_iov));
>>>> -       BUILD_BUG_ON(offsetof(struct vringh_kiov, iov) !=
>>>> -                    offsetof(struct vringh_iov, iov));
>>>> -       BUILD_BUG_ON(offsetof(struct vringh_kiov, i) !=
>>>> -                    offsetof(struct vringh_iov, i));
>>>> -       BUILD_BUG_ON(offsetof(struct vringh_kiov, used) !=
>>>> -                    offsetof(struct vringh_iov, used));
>>>> -       BUILD_BUG_ON(offsetof(struct vringh_kiov, max_num) !=
>>>> -                    offsetof(struct vringh_iov, max_num));
>>>> -       BUILD_BUG_ON(sizeof(struct iovec) != sizeof(struct kvec));
>>>> -       BUILD_BUG_ON(offsetof(struct iovec, iov_base) !=
>>>> -                    offsetof(struct kvec, iov_base));
>>>> -       BUILD_BUG_ON(offsetof(struct iovec, iov_len) !=
>>>> -                    offsetof(struct kvec, iov_len));
>>>> -       BUILD_BUG_ON(sizeof(((struct iovec *)NULL)->iov_base)
>>>> -                    != sizeof(((struct kvec *)NULL)->iov_base));
>>>> -       BUILD_BUG_ON(sizeof(((struct iovec *)NULL)->iov_len)
>>>> -                    != sizeof(((struct kvec *)NULL)->iov_len));
>>>> -
>>>>          *head = err;
>>>>          err = __vringh_iov(vrh, *head, (struct vringh_kiov *)riov,
>>>>                             (struct vringh_kiov *)wiov,
>>>> @@ -740,14 +720,14 @@ int vringh_getdesc_user(struct vringh *vrh,
>>>>   EXPORT_SYMBOL(vringh_getdesc_user);
>>>>
>>>>   /**
>>>> - * vringh_iov_pull_user - copy bytes from vring_iov.
>>>> + * vringh_iov_pull_user - copy bytes from vring_kiov.
>>>>    * @riov: the riov as passed to vringh_getdesc_user() (updated as we consume)
>>>>    * @dst: the place to copy.
>>>>    * @len: the maximum length to copy.
>>>>    *
>>>>    * Returns the bytes copied <= len or a negative errno.
>>>>    */
>>>> -ssize_t vringh_iov_pull_user(struct vringh_iov *riov, void *dst, size_t len)
>>>> +ssize_t vringh_iov_pull_user(struct vringh_kiov *riov, void *dst, size_t len)
>>>>   {
>>>>          return vringh_iov_xfer(NULL, (struct vringh_kiov *)riov,
>>>>                                 dst, len, xfer_from_user);
>>>> @@ -755,14 +735,14 @@ ssize_t vringh_iov_pull_user(struct vringh_iov *riov, void *dst, size_t len)
>>>>   EXPORT_SYMBOL(vringh_iov_pull_user);
>>>>
>>>>   /**
>>>> - * vringh_iov_push_user - copy bytes into vring_iov.
>>>> + * vringh_iov_push_user - copy bytes into vring_kiov.
>>>>    * @wiov: the wiov as passed to vringh_getdesc_user() (updated as we consume)
>>>>    * @src: the place to copy from.
>>>>    * @len: the maximum length to copy.
>>>>    *
>>>>    * Returns the bytes copied <= len or a negative errno.
>>>>    */
>>>> -ssize_t vringh_iov_push_user(struct vringh_iov *wiov,
>>>> +ssize_t vringh_iov_push_user(struct vringh_kiov *wiov,
>>>>                               const void *src, size_t len)
>>>>   {
>>>>          return vringh_iov_xfer(NULL, (struct vringh_kiov *)wiov,
>>>> diff --git a/include/linux/vringh.h b/include/linux/vringh.h
>>>> index 1991a02c6431..733d948e8123 100644
>>>> --- a/include/linux/vringh.h
>>>> +++ b/include/linux/vringh.h
>>>> @@ -79,18 +79,6 @@ struct vringh_range {
>>>>          u64 offset;
>>>>   };
>>>>
>>>> -/**
>>>> - * struct vringh_iov - iovec mangler.
>>>> - *
>>>> - * Mangles iovec in place, and restores it.
>>>> - * Remaining data is iov + i, of used - i elements.
>>>> - */
>>>> -struct vringh_iov {
>>>> -       struct iovec *iov;
>>>> -       size_t consumed; /* Within iov[i] */
>>>> -       unsigned i, used, max_num;
>>>> -};
>>>> -
>>>>   /**
>>>>    * struct vringh_kiov - kvec mangler.
>>>>    *
>>>> @@ -113,44 +101,19 @@ int vringh_init_user(struct vringh *vrh, u64 features,
>>>>                       vring_avail_t __user *avail,
>>>>                       vring_used_t __user *used);
>>>>
>>>> -static inline void vringh_iov_init(struct vringh_iov *iov,
>>>> -                                  struct iovec *iovec, unsigned num)
>>>> -{
>>>> -       iov->used = iov->i = 0;
>>>> -       iov->consumed = 0;
>>>> -       iov->max_num = num;
>>>> -       iov->iov = iovec;
>>>> -}
>>>> -
>>>> -static inline void vringh_iov_reset(struct vringh_iov *iov)
>>>> -{
>>>> -       iov->iov[iov->i].iov_len += iov->consumed;
>>>> -       iov->iov[iov->i].iov_base -= iov->consumed;
>>>> -       iov->consumed = 0;
>>>> -       iov->i = 0;
>>>> -}
>>>> -
>>>> -static inline void vringh_iov_cleanup(struct vringh_iov *iov)
>>>> -{
>>>> -       if (iov->max_num & VRINGH_IOV_ALLOCATED)
>>>> -               kfree(iov->iov);
>>>> -       iov->max_num = iov->used = iov->i = iov->consumed = 0;
>>>> -       iov->iov = NULL;
>>>> -}
>>>> -
>>>>   /* Convert a descriptor into iovecs. */
>>>>   int vringh_getdesc_user(struct vringh *vrh,
>>>> -                       struct vringh_iov *riov,
>>>> -                       struct vringh_iov *wiov,
>>>> +                       struct vringh_kiov *riov,
>>>> +                       struct vringh_kiov *wiov,
>>>>                          bool (*getrange)(struct vringh *vrh,
>>>>                                           u64 addr, struct vringh_range *r),
>>>>                          u16 *head);
>>>>
>>>>   /* Copy bytes from readable vsg, consuming it (and incrementing wiov->i). */
>>>> -ssize_t vringh_iov_pull_user(struct vringh_iov *riov, void *dst, size_t len);
>>>> +ssize_t vringh_iov_pull_user(struct vringh_kiov *riov, void *dst, size_t len);
>>>>
>>>>   /* Copy bytes into writable vsg, consuming it (and incrementing wiov->i). */
>>>> -ssize_t vringh_iov_push_user(struct vringh_iov *wiov,
>>>> +ssize_t vringh_iov_push_user(struct vringh_kiov *wiov,
>>>>                               const void *src, size_t len);
>>>>
>>>>   /* Mark a descriptor as used. */
>>>> --
>>>> 2.25.1
>>>>
>> Best,
>> Shunsuke
>>