On Thu, May 30, 2024 at 2:12 PM Alex Bennée <alex.ben...@linaro.org> wrote:
> Cord Amfmgm <dmamf...@gmail.com> writes:
>
> > On Thu, May 30, 2024 at 3:33 AM Alex Bennée <alex.ben...@linaro.org>
> wrote:
> >
> > Cord Amfmgm <dmamf...@gmail.com> writes:
> >
> > > On Tue, May 28, 2024 at 11:32 AM Peter Maydell <
> peter.mayd...@linaro.org> wrote:
> > >
> > > On Tue, 28 May 2024 at 16:37, Cord Amfmgm <dmamf...@gmail.com>
> wrote:
> > > >
> > > > On Tue, May 28, 2024 at 9:03 AM Peter Maydell <
> peter.mayd...@linaro.org> wrote:
> > > >>
> > > >> On Mon, 20 May 2024 at 23:24, Cord Amfmgm <dmamf...@gmail.com>
> wrote:
> > > >> > On Mon, May 20, 2024 at 12:05 PM Peter Maydell <
> peter.mayd...@linaro.org> wrote:
> > <snip>
> > > >> > And here's an example buffer of length 0 -- you probably
> already know what I'm going to do here:
> > > >> >
> > > >> > char buf[0];
> > > >> > char * CurrentBufferPointer = &buf[0];
> > > >> > char * BufferEnd = &buf[-1]; // "address of the last byte in
> the buffer"
> > > >> > // The OHCI Host Controller than advances CurrentBufferPointer
> like this: CurrentBufferPointer += 0
> > > >> > // After the transfer:
> > > >> > // CurrentBufferPointer = &buf[0];
> > > >> > // BufferEnd = &buf[-1];
> > > >>
> > > >> Right, but why do you think this is valid, rather than
> > > >> being a guest software bug? My reading of the spec is that it's
> > > >> pretty clear about how to say "zero length buffer", and this
> > > >> isn't it.
> > > >>
> > > >> Is there some real-world guest OS that programs the OHCI
> > > >> controller this way that we're trying to accommodate?
> > > >
> > > >
> > > > qemu versions 4.2 and before allowed this behavior.
> > >
> > > So? That might just mean we had a bug and we fixed it.
> > > 4.2 is a very old version of QEMU and nobody seems to have
> > > complained in the four years since we released 5.0 about this,
> > > which suggests that generally guest OS drivers don't try
> > > to send zero-length buffers in this way.
> > >
> > > > I don't think it's valid to ask for a *popular* guest OS as a
> proof-of-concept because I'm not an expert on those.
> > >
> > > I didn't ask for "popular"; I asked for "real-world".
> > > What is the actual guest code you're running that falls over
> > > because of the behaviour change?
> > >
> > > More generally, why do you want this behaviour to be
> > > changed? Reasonable reasons might include:
> > > * we're out of spec based on reading the documentation
> > > * you're trying to run some old Windows VM/QNX/etc image,
> > > and it doesn't work any more
> > > * all the real hardware we tested behaves this way
> > >
> > > But don't necessarily include:
> > > * something somebody wrote and only tested on QEMU happens to
> > > assume the old behaviour rather than following the hw spec
> > >
> > > QEMU occasionally works around guest OS bugs, but only as
> > > when we really have to. It's usually better to fix the
> > > bug in the guest.
> > >
> > > It's not, and I've already demonstrated that real hardware is
> consistent with the fix in this patch.
> > >
> > > Please check your tone.
> >
> > I don't think that is a particularly helpful comment for someone who is
> > taking the time to review your patches. Reading through the thread I
> > didn't see anything that said this is how real HW behaves but I may well
> > have missed it. However you have a number of review comments to address
> > so I suggest you spin a v2 of the series to address them and outline the
> > reason to accept an out of spec transaction.
> >
> > I did a rework of the patch -- see my email from May 20, quoted below --
> and I was under the impression it addressed all the
> > review comments. Did I miss something? I apologize if I did.
>
> Ahh I see - I'd only seen this thread continue so wasn't aware a new
> version had been posted. For future patches consider using -vN when
> sending them so we can clearly see a new revision is available.
>
> >
> >> index acd6016980..71b54914d3 100644
> >> --- a/hw/usb/hcd-ohci.c
> >> +++ b/hw/usb/hcd-ohci.c
> >> @@ -941,8 +941,8 @@ static int ohci_service_td(OHCIState *ohci, struct
> ohci_ed *ed)
> >> if ((td.cbp & 0xfffff000) != (td.be & 0xfffff000)) {
> >> len = (td.be & 0xfff) + 0x1001 - (td.cbp & 0xfff);
> >> } else {
> >> - if (td.cbp > td.be) {
> >> - trace_usb_ohci_iso_td_bad_cc_overrun(td.cbp, td.be);
> >> + if (td.cbp - 1 > td.be) { /* rely on td.cbp != 0 */
> >
> >> Reading through the thread I didn't see anything that said this is how
> real HW behaves but I may well have missed it.
> >
> > This is what I wrote regarding real HW:
> >
> > Results are:
> >
> > qemu 4.2 | qemu HEAD | actual HW
> > ------------+------------+------------
> > works fine | ohci_die() | works fine
> >
> > Would additional verification of the actual HW be useful?
> >
> > Peter posted the following which is more specific than "qemu 4.2" -- I
> agree this is most likely the qemu commit where this
> > thread is focused:
> >
> >> Almost certainly this was commit 1328fe0c32d54 ("hw: usb: hcd-ohci:
> >> check len and frame_number variables"), which added these bounds
> >> checks. Prior to that we did no bounds checking at all, which
> >> meant that we permitted cbp=be+1 to mean a zero length, but also
> >> that we permitted the guest to overrun host-side buffers by
> >> specifying completely bogus cbp and be values. The timeframe is
> >> more or less right (2020), at least.
> >>
> >> -- PMM
> >
> > Where does the conversation go from here? I'm under the impression I
> have provided objective answers to all the questions
> > and resolved all review comments on the code. I receive the feedback
> > that I missed something - please restate the question?
>
> I can see patch 1/2 has been queued and 2/2 is still outstanding. I'm
> having trouble finding the referenced entry in the OHCI spec. The only
> one I can see is Release 1.1, January 6th, 2000 and that doesn't have a
> section 4.3.1.2.
>
> I think discussion should continue on that thread.
>
Yes, agreed.
>
> --
> Alex Bennée
> Virtualisation Tech Lead @ Linaro
>
