Re: [Qemu-devel] [RFC PATCH v2 4/6] hw/pci: introduce bridge-only vendor-specific capability to provide some hints to firmware

[Michael S. Tsirkin]

On Thu, Jul 27, 2017 at 12:54:07AM +0300, Alexander Bezzubikov wrote:
> 2017-07-26 22:43 GMT+03:00 Michael S. Tsirkin <address@hidden>:
> > On Sun, Jul 23, 2017 at 01:15:41AM +0300, Aleksandr Bezzubikov wrote:
> >> On PCI init PCI bridges may need some
> >> extra info about bus number to reserve, IO, memory and
> >> prefetchable memory limits. QEMU can provide this
> >> with special
> >
> > with a special
> >
> >> vendor-specific PCI capability.
> >>
> >> Sizes of limits match ones from
> >> PCI Type 1 Configuration Space Header,
> >> number of buses to reserve occupies only 1 byte
> >> since it is the size of Subordinate Bus Number register.
> >>
> >> Signed-off-by: Aleksandr Bezzubikov <address@hidden>
> >> ---
> >>  hw/pci/pci_bridge.c         | 27 +++++++++++++++++++++++++++
> >>  include/hw/pci/pci_bridge.h | 18 ++++++++++++++++++
> >>  2 files changed, 45 insertions(+)
> >>
> >> diff --git a/hw/pci/pci_bridge.c b/hw/pci/pci_bridge.c
> >> index 720119b..8ec6c2c 100644
> >> --- a/hw/pci/pci_bridge.c
> >> +++ b/hw/pci/pci_bridge.c
> >> @@ -408,6 +408,33 @@ void pci_bridge_map_irq(PCIBridge *br, const char* 
> >> bus_name,
> >>      br->bus_name = bus_name;
> >>  }
> >>
> >> +
> >> +int pci_bridge_help_cap_init(PCIDevice *dev, int cap_offset,
> >
> > help? should be qemu_cap_init?
> >
> >> +                              uint8_t bus_reserve, uint32_t io_limit,
> >> +                              uint16_t mem_limit, uint64_t pref_limit,
> >> +                              Error **errp)
> >> +{
> >> +    size_t cap_len = sizeof(PCIBridgeQemuCap);
> >> +    PCIBridgeQemuCap cap;
> >
> > This leaks info to guest. You want to init all fields here:
> >
> > cap = {
> >  .len = ....
> > };
> 
> I surely can do this for len field, but as Laszlo proposed
> we can use mutually exclusive fields,
> e.g. pref_32 and pref_64, the only way I have left
> is to use ternary operator (if we surely need this
> big initializer). Keeping some if's would look better,
> I think.
> 
> >
> >> +
> >> +    cap.len = cap_len;
> >> +    cap.bus_res = bus_reserve;
> >> +    cap.io_lim = io_limit & 0xFF;
> >> +    cap.io_lim_upper = io_limit >> 8 & 0xFFFF;
> >> +    cap.mem_lim = mem_limit;
> >> +    cap.pref_lim = pref_limit & 0xFFFF;
> >> +    cap.pref_lim_upper = pref_limit >> 16 & 0xFFFFFFFF;
> >
> > Please use pci_set_word etc or cpu_to_leXX.
> >
> 
> Since now we've decided to avoid fields separation into <field> + 
> <field_upper>,
> this bitmask along with pci_set_word are no longer needed.
> 
> > I think it's easiest to replace struct with a set of macros then
> > pci_set_word does the work for you.
> >
> 
> I don't really want to use macros here because structure
> show us the whole capability layout and this can
> decrease documenting efforts. More than that,
> memcpy usage is very convenient here, and I wouldn't like
> to lose it.
> 
> >
> >> +
> >> +    int offset = pci_add_capability(dev, PCI_CAP_ID_VNDR,
> >> +                                    cap_offset, cap_len, errp);
> >> +    if (offset < 0) {
> >> +        return offset;
> >> +    }
> >> +
> >> +    memcpy(dev->config + offset + 2, (char *)&cap + 2, cap_len - 2);
> >
> > +2 is yacky. See how virtio does it:
> >
> >     memcpy(dev->config + offset + PCI_CAP_FLAGS, &cap->cap_len,
> >            cap->cap_len - PCI_CAP_FLAGS);
> >
> >
> 
> OK.
> 
> >> +    return 0;
> >> +}
> >> +
> >>  static const TypeInfo pci_bridge_type_info = {
> >>      .name = TYPE_PCI_BRIDGE,
> >>      .parent = TYPE_PCI_DEVICE,
> >> diff --git a/include/hw/pci/pci_bridge.h b/include/hw/pci/pci_bridge.h
> >> index ff7cbaa..c9f642c 100644
> >> --- a/include/hw/pci/pci_bridge.h
> >> +++ b/include/hw/pci/pci_bridge.h
> >> @@ -67,4 +67,22 @@ void pci_bridge_map_irq(PCIBridge *br, const char* 
> >> bus_name,
> >>  #define  PCI_BRIDGE_CTL_DISCARD_STATUS       0x400   /* Discard timer 
> >> status */
> >>  #define  PCI_BRIDGE_CTL_DISCARD_SERR 0x800   /* Discard timer SERR# 
> >> enable */
> >>
> >> +typedef struct PCIBridgeQemuCap {
> >> +    uint8_t id;     /* Standard PCI capability header field */
> >> +    uint8_t next;   /* Standard PCI capability header field */
> >> +    uint8_t len;    /* Standard PCI vendor-specific capability header 
> >> field */
> >> +    uint8_t bus_res;
> >> +    uint32_t pref_lim_upper;
> >
> > Big endian? Ugh.
> >
> 
> Agreed, and this's gonna to disappear with
> the new layout.
> 
> >> +    uint16_t pref_lim;
> >> +    uint16_t mem_lim;
> >
> > I'd say we need 64 bit for memory.
> >
> 
> Why? Non-prefetchable MEMORY_LIMIT register is 16 bits long.

Hmm ok, but e.g. for io there are bridges that have extra registers
to specify non-standard non-aligned registers.

> >> +    uint16_t io_lim_upper;
> >> +    uint8_t io_lim;
> >> +    uint8_t padding;
> >
> > IMHO each type should have a special "don't care" flag
> > that would mean "I don't know".
> >
> >
> 
> Don't know what? Now 0 is an indicator to do nothing with this field.

In that case how do you say "don't allocate any memory"?


> >> +} PCIBridgeQemuCap;
> >
> > You don't really need this struct in the header. And pls document all 
> > fields.
> >
> >> +
> >> +int pci_bridge_help_cap_init(PCIDevice *dev, int cap_offset,
> >> +                              uint8_t bus_reserve, uint32_t io_limit,
> >> +                              uint16_t mem_limit, uint64_t pref_limit,
> >> +                              Error **errp);
> >> +
> >>  #endif /* QEMU_PCI_BRIDGE_H */
> >> --
> >> 2.7.4
> 
> 
> 
> --
> Alexander Bezzubikov