Re: [Xen-devel] [PATCH v3 07/15] argo: implement the register op

[Christopher Clark <christopher.w.clark@xxxxxxxxx>]

On Tue, Jan 15, 2019 at 12:36 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>
> >>> On 15.01.19 at 08:56, <christopher.w.clark@xxxxxxxxx> wrote:
> > On Mon, Jan 14, 2019 at 6:19 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >> >>> On 07.01.19 at 08:42, <christopher.w.clark@xxxxxxxxx> wrote:
> >> > +    /*
> >> > +     * A ring must be large enough to transmit messages, so requires 
> >> > space for:
> >> > +     * * 1 message header, plus
> >> > +     * * 1 payload slot (payload is always rounded to a multiple of 16 
> >> > bytes)
> >> > +     *   for the message payload to be written into, plus
> >> > +     * * 1 more slot, so that the ring cannot be filled to capacity 
> >> > with a
> >> > +     *   single message -- see the logic in ringbuf_insert -- allowing 
> >> > for this
> >> > +     *   ensures that there can be space remaining when a message is 
> >> > present.
> >> > +     * The above determines the minimum acceptable ring size.
> >> > +     */
> >> > +    if ( (reg.len < (sizeof(struct xen_argo_ring_message_header)
> >> > +                      + ROUNDUP_MESSAGE(1) + ROUNDUP_MESSAGE(1))) ||
> >>
> >> These two summands don't look to fulfill the "cannot be filled to
> >> capacity" constraint the comment describes, as (aiui) messages
> >> can be larger than 16 bytes. What's the deal?
> >
> > This is intended to be about bound checking reg.len against a minimum
> > size: the smallest ring that you can fit a message onto, as determined
> > by the logic in ringbuf_insert. The smallest message you can send is:
> > sizeof(struct xen_argo_ring_message_header) + ROUNDUP_MESSAGE(1)
> >
> > and then ringbuf_insert won't accept a message unless there is (at
> > least) ROUNDUP_MESSAGE(1) space remaining, so that, plus the smallest
> > message size, is the smallest viable ring. There's no point accepting
> > registration of a ring smaller than that.
> >
> > You're right that messages can be larger than 16 bytes, but they can
> > only be sent to rings that are larger than that minimum - on a minimum
> > sized ring, they'll be rejected by sendv.
>
> So perhaps the comment wants to say "... cannot be filled to capacity
> with a single, minimum size message"?

ack, done.

Christopher

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