Re: [PATCH] mini-os: xenbus: support large messages

[Juergen Gross <jgross@xxxxxxxx>]

On 15.09.21 13:20, Samuel Thibault wrote:
> Juergen Gross, le mer. 15 sept. 2021 12:48:44 +0200, a ecrit:
> > On 15.09.21 01:17, Samuel Thibault wrote:
> > > > +        prod = xenstore_buf->rsp_prod;
> > > > +        DEBUG("Rsp_cons %d, rsp_prod %d.\n", xenstore_buf->rsp_cons, prod);
> > > > +        size = min(len - off, prod - xenstore_buf->rsp_cons);
> > > > +        memcpy_from_ring(xenstore_buf->rsp, buf + off,
> > > > +                         MASK_XENSTORE_IDX(xenstore_buf->rsp_cons), size);
> > > > +        off += size;
> > > > +        notify = (xenstore_buf->rsp_cons + XENSTORE_RING_SIZE ==
> > > > +                  xenstore_buf->rsp_prod);
> > > This looks odd to me?  We want to notify as soon as the ring is empty,
> > > which can happen at any place in the ring right?
> > No, we want to notify if the ring was full and is about to gain some
> > space again, as the other side was probably not able to put all data
> > in and is now waiting for more space to become available.
> Ok, that said, the producer may fill the ring between this test and
> the rsp_cons update, and thus the producer will go sleep and here the
> consumer will not notice it and thus never notify it.
>
> So we really need to make the test after the rsp_cons update, like Linux
> does:
>
> > > Linux' code uses (intf->rsp_prod - cons >= XENSTORE_RING_SIZE), *after*
> > > the rsp_cons increase.
Oh, you are right, of course. How could I overlook this?

Thanks,


Juergen

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