Re: [PATCH v11 2/7] KVM: stats: Add fd-based API to read binary stats data

[Jing Zhang <jingzhangos@google.com>]

On Fri, Jun 18, 2021 at 1:57 AM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> Minor comment nits:
>
> On Fri, Jun 18, 2021 at 04:48:14AM +0000, Jing Zhang wrote:
> > +/*
> > + * Common vm/vcpu stats read function to userspace.
>
> Should you use a real kernel-doc style here?  You almost are, might as
> well do it "right" :)
>
Will fix that.
> > + * @id: identification string of the stats
> > + * @header: stats header for a vm or a vcpu
> > + * @desc: start address of an array of stats descriptors for a vm or a vcpu
> > + * @stats: start address of stats data block for a vm or a vcpu
> > + * @size_stats: the size of stats data block pointed by @stats
> > + * @user_buffer: start address of userspace buffer
> > + * @size: requested read size from userspace
> > + * @offset: the start position from which the content will be read for the
> > + *          corresponding vm or vcp file descriptor
> > + *
> > + * The file content of a vm/vcpu file descriptor is now defined as below:
> > + * +-------------+
> > + * |   Header    |
> > + * +-------------+
> > + * | Descriptors |
> > + * +-------------+
> > + * | Stats Data  |
> > + * +-------------+
>
> Where is the "header id string"?  In the header?
>
Yes, the id string is in the header.
> > + * Although this function allows userspace to read any amount of data (as long
> > + * as in the limit) from any position, the typical usage would follow below
> > + * steps:
> > + * 1. Read header from offset 0. Get the offset of descriptors and stats data
> > + *    and some other necessary information. This is a one-time work for the
> > + *    lifecycle of the corresponding vm/vcpu stats fd.
> > + * 2. Read descriptors from its offset and discover all the stats by parsing
> > + *    descriptors. This is a one-time work for the lifecycle of the
> > + *    corresponding vm/vcpu stats fd.
> > + * 3. Periodically read stats data from its offset.
>
> You forgot "2.5.  rewind fd pointer position", see below...
>
Sure, will clarify that.
> > + */
> > +ssize_t kvm_stats_read(char *id, struct kvm_stats_header *header,
> > +             struct _kvm_stats_desc *desc, void *stats, size_t size_stats,
> > +             char __user *user_buffer, size_t size, loff_t *offset)
> > +{
> > +     ssize_t len;
> > +     ssize_t copylen;
> > +     ssize_t remain = size;
> > +     size_t size_desc;
> > +     size_t size_header;
> > +     void *src;
> > +     loff_t pos = *offset;
> > +     char __user *dest = user_buffer;
> > +
> > +     size_header = sizeof(*header);
> > +     size_desc = header->count * sizeof(*desc);
> > +
> > +     len = KVM_STATS_ID_MAXLEN + size_header + size_desc + size_stats - pos;
> > +     len = min(len, remain);
> > +     if (len <= 0)
> > +             return 0;
> > +     remain = len;
> > +
> > +     /* Copy kvm stats header.
> > +      * The header is the first block of content userspace usually read out.
> > +      * The pos is 0 and the copylen and remain would be the size of header.
> > +      * The copy of the header would be skipped if offset is larger than the
> > +      * size of header. That usually happens when userspace reads stats
> > +      * descriptors and stats data.
> > +      */
>
> Looks like this is the networking "style" of multi-line comments, not
> the rest of the kernel.  You might want to fix this up to be the normal
> style which would be:
>
>         /*
>          * Copy kvm stats header.
>          * The header is the first block of content userspace usually read out.
>          * The pos is 0 and the copylen and remain would be the size of header.
>          * The copy of the header would be skipped if offset is larger than the
>          * size of header. That usually happens when userspace reads stats
>          * descriptors and stats data.
>          */
>
> I do not know how picky the kvm maintainers are about this, that's up to
> them :)
>
>
Will fix it.
> > +     copylen = size_header - pos;
> > +     copylen = min(copylen, remain);
> > +     if (copylen > 0) {
> > +             src = (void *)header + pos;
> > +             if (copy_to_user(dest, src, copylen))
> > +                     return -EFAULT;
> > +             remain -= copylen;
> > +             pos += copylen;
> > +             dest += copylen;
> > +     }
> > +
> > +     /* Copy kvm stats header id string.
> > +      * The id string is unique for every vm/vcpu, which is stored in kvm
> > +      * and kvm_vcpu structure.
> > +      */
>
> This header too is skipped if necessary, so you should say that as well.
>
>
Sure, will clarify that.
> > +     copylen = size_header + KVM_STATS_ID_MAXLEN - pos;
> > +     copylen = min(copylen, remain);
> > +     if (copylen > 0) {
> > +             src = id + pos - size_header;
> > +             if (copy_to_user(dest, src, copylen))
> > +                     return -EFAULT;
> > +             remain -= copylen;
> > +             pos += copylen;
> > +             dest += copylen;
> > +     }
> > +
> > +     /* Copy kvm stats descriptors.
> > +      * The descriptors copy would be skipped in the typical case that
> > +      * userspace periodically read stats data, since the pos would be
> > +      * greater than the end address of descriptors
> > +      * (header->header.desc_offset + size_desc) causing copylen <= 0.
> > +      */
>
> But you say that it is skipped here.
>
> > +     copylen = header->desc_offset + size_desc - pos;
> > +     copylen = min(copylen, remain);
> > +     if (copylen > 0) {
> > +             src = (void *)desc + pos - header->desc_offset;
> > +             if (copy_to_user(dest, src, copylen))
> > +                     return -EFAULT;
> > +             remain -= copylen;
> > +             pos += copylen;
> > +             dest += copylen;
> > +     }
> > +
> > +     /* Copy kvm stats values */
> > +     copylen = header->data_offset + size_stats - pos;
> > +     copylen = min(copylen, remain);
> > +     if (copylen > 0) {
> > +             src = stats + pos - header->data_offset;
>
> This lets you sync to the end of the header and read just the stats, but
> does that mean that userspace keeps needing to "rewind" back to the end
> of the header to read the stats again?
>
> Or can it just keep reading off the end of the previous read?
>
> It's not quite obvious here, and I mention that above in step "2.5", so
> maybe I am wrong, which is fine, but then I'm confused :)
Userspace needs to rewind back to read the stats again or just use pread
as Paolo mentioned and that's used in the testcase.
>
>
> thanks,
>
> greg k-h