Re: [Qemu-devel] [PATCH v4 2/3] pci: add QEMU-specific PCI capability structure

[Marcel Apfelbaum]

On 07/08/2017 19:02, Alexander Bezzubikov wrote:
> 2017-08-07 18:52 GMT+03:00 Marcel Apfelbaum <address@hidden>:
> > On 05/08/2017 23:29, Aleksandr Bezzubikov wrote:
> > > On PCI init PCI bridge devices may need some
> > > extra info about bus number to reserve, IO, memory and
> > > prefetchable memory limits. QEMU can provide this
> > > with special vendor-specific PCI capability.
> > >
> > > This capability is intended to be used only
> > > for Red Hat PCI bridges, i.e. QEMU cooperation.
> > >
> > > Signed-off-by: Aleksandr Bezzubikov <address@hidden>
> > > ---
> > >    src/fw/dev-pci.h | 50 ++++++++++++++++++++++++++++++++++++++++++++++++++
> > >    1 file changed, 50 insertions(+)
> > >    create mode 100644 src/fw/dev-pci.h
> > >
> > > diff --git a/src/fw/dev-pci.h b/src/fw/dev-pci.h
> > > new file mode 100644
> > > index 0000000..2c8ddb0
> >
> >
> > Hi Aleksandr,
>
> Hi Marcel,
>
> > > --- /dev/null
> > > +++ b/src/fw/dev-pci.h
> > > @@ -0,0 +1,50 @@
> > > +#ifndef _PCI_CAP_H
> > > +#define _PCI_CAP_H
> > > +
> > > +#include "types.h"
> > > +
> > > +/*
> > > +
> >
> >
> > Please use the standard comment:
> >
> > /*
> >   *
> >   *
> >   */
> >
> >
> > > +QEMU-specific vendor(Red Hat)-specific capability.
> > > +It's intended to provide some hints for firmware to init PCI devices.
> > > +
> > > +Its structure is shown below:
> > > +
> > > +Header:
> > > +
> > > +u8 id;       Standard PCI Capability Header field
> > > +u8 next;     Standard PCI Capability Header field
> > > +u8 len;      Standard PCI Capability Header field
> > > +u8 type;     Red Hat vendor-specific capability type:
> > > +               now only REDHAT_CAP_TYP_QEMU=1 exists
> >
> >
> > Typo o the line before, but I think you don't need it there.
> >
> > > +Data:
> > > +
> > > +u32 bus_res;            minimum bus number to reserve;
> > > +                        this is necessary for PCI Express Root Ports
> > > +                        to support PCIE-to-PCI bridge hotplug
> >
> >
> > I would add a broader class of usage:
> >       necessary for nesting PCI bridges hotplug.
> >
> > > +u64 io;                 IO space to reserve
> > > +u64 mem;                non-prefetchable memory space to reserve
> > > +u64 prefetchable_mem;   prefetchable memory space to reserve
> > > +
> >
> >
> > Layout looks good to me.
> >
> > > +If any field value in Data section is -1,
> > > +it means that such kind of reservation
> > > +is not needed and must be ignored.
> > > +
> >
> >
> > -1 is not a valid value for unsigned fields, you may
> > want to say 0xff..f or some other way.
>
> I meant cast to unsigned here (because we still use unsigned types),
> but if it can mislead someone I will change this.
We should not document signed values for unsigned fields,
even if the reason is "best practices."

> > > +*/
> > > +
> > > +/* Offset of vendor-specific capability type field */
> > > +#define PCI_CAP_REDHAT_TYPE  3
> >
> >
> > May I ask why why '3'? I am not against it, I just
> > want to understand the number.
>
> This is actually an offset to this field

OK, so it should end with 'offset' to be clear.
I was mislead.

> > > +
> > > +/* List of valid Red Hat vendor-specific capability types */
> > > +#define REDHAT_CAP_TYPE_QEMU    1
> >
> >
> > I think I pointed this in another thread, the name is
> > too vague, please change it to something like:
> >     REDHAT_CAP_RES_RESERVE_QEMU
> > that narrows down the intend.
>
> What does the first 'RES' mean?

Resource.

I don't mind you change it how you seem fit,
just make it clear what it does. Is about
resource reserving, not a general capability.


Thanks,
Marcel

> > > +
> > > +
> > > +/* Offsets of QEMU capability fields */
> > > +#define QEMU_PCI_CAP_BUS_RES        4
> > > +#define QEMU_PCI_CAP_LIMITS_OFFSET  8
> > > +#define QEMU_PCI_CAP_IO             8
> > > +#define QEMU_PCI_CAP_MEM            16
> > > +#define QEMU_PCI_CAP_PREF_MEM       24
> > > +#define QEMU_PCI_CAP_SIZE           32
> > > +
> > > +#endif /* _PCI_CAP_H */
> >
> > The layout looks good to me.
> >
> > Thanks,
> > Marcel