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" <mst@redhat.com>]

On Mon, Jul 31, 2017 at 01:06:23PM +0300, Marcel Apfelbaum wrote:
> On 29/07/2017 2:12, Michael S. Tsirkin wrote:
> > On Thu, Jul 27, 2017 at 12:39:54PM +0300, Marcel Apfelbaum wrote:
> > > On 27/07/2017 2:28, Michael S. Tsirkin wrote:
> > > > On Thu, Jul 27, 2017 at 12:54:07AM +0300, Alexander Bezzubikov wrote:
> > > > > 2017-07-26 22:43 GMT+03:00 Michael S. Tsirkin <mst@redhat.com>:
> > > > > > 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 <zuban32s@gmail.com>
> > > > > > > ---
> > > > > > >    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"?
> > > > 
> > > 
> > > We can keep the MEM/Limit registers read-only for such cases,
> > > as they are optional registers.
> > > 
> > > Thanks,
> > > Marcel
> > 
> 
> Hi Michael,
> 
> 
> > I don't believe they are - from the spec (1.2):
> > 	The Memory Base and Memory Limit registers are both required registers
> > 
> > Rest of ranges are indeed optional.
> > 
> 
> 
> Oops, my mistake, I looked at prefetchable memory, sorry for the mix-up.
> I wonder what would be the use case to disable IOMEM.
> 
> Anyway, we can have a "-1" default value giving the hint: do not reserve
> anything, but in this case we should use it also for IO/prefetch mem.
> The problem is, the guest OS can try to allocate IO space even if
> the firmware didn't; Linux does it today.
> Having IO-base/IO-limit registers read-only will solve the problem.

Makes sense.

> What am I getting at is we would have now 2 ways to solve the same
> problem and the hint would be inferior.
> 
> I would use the hint to reserve a specific range and read only registers
> to disable the range. IOMEM cannot be disabled in this way, but
> should not bother us.
> 
> Thanks,
> Marcel

I think Laszlo said that RO as an interface to indicate optional
range is tricky to use.

> > 
> > > > 
> > > > > > > +} 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