Re: [Qemu-devel] [PATCH V7 06/11] pci.c: Add pci_check_bar_overlap

[Anthony PERARD]

On Sun, Feb 19, 2012 at 13:35, Michael S. Tsirkin <address@hidden> wrote:
> On Fri, Feb 17, 2012 at 05:08:40PM +0000, Anthony PERARD wrote:
>> From: Yuji Shimada <address@hidden>
>>
>> This function helps Xen PCI Passthrough device to check for overlap.
>>
>> Signed-off-by: Yuji Shimada <address@hidden>
>> Signed-off-by: Anthony PERARD <address@hidden>
>
> As far as I can see, this scans the bus a specific
> device is on, looking for devices who have conflicting
> BARs. Returns 1 if found, 0 if not.
>
> Not sure what would devices do with this information, really:
> patches posted just print out a warning which does not seem
> too useful.

This function is just here to print a warning in case an overlap
happen, but we would like too keep this warning.

> Just FYI, if you decided to e.g. disable device in
> such a case that would be wrong: it is legal to have overlapping BARs as
> long as there are no accesses.
> So a legal thing for a guest to do is:
>
> Assign BAR1 = abcde
> Assign BAR2 = abcde <- overlaps temporarily
> Assign BAR1 = 12345
>
> And this means that you can't just check your device
> has no conflicts when your device is touched:
> it needs to be checked each time mappings are updated.
>
>> ---
>>  hw/pci.c |   47 +++++++++++++++++++++++++++++++++++++++++++++++
>>  hw/pci.h |    3 +++
>>  2 files changed, 50 insertions(+), 0 deletions(-)
>>
>> diff --git a/hw/pci.c b/hw/pci.c
>> index 678a8c1..75d6529 100644
>> --- a/hw/pci.c
>> +++ b/hw/pci.c
>> @@ -1985,6 +1985,53 @@ MemoryRegion *pci_address_space_io(PCIDevice *dev)
>>      return dev->bus->address_space_io;
>>  }
>>
>> +int pci_check_bar_overlap(PCIDevice *dev,
>> +                          pcibus_t addr, pcibus_t size, uint8_t type)
>
> This lacks comments documentng what this does, parameters, return
> values, etc.

I'll fix that.

>> +{
>> +    PCIBus *bus = dev->bus;
>> +    PCIDevice *devices = NULL;
>> +    PCIIORegion *r;
>> +    int i, j;
>> +    int rc = 0;
>> +
>> +    /* check Overlapped to Base Address */
>
> Weird use of upper case. Intentional?
> Also - what does this comment mean?

Don't know, I will remove the comment, it is useless.

>> +    for (i = 0; i < ARRAY_SIZE(bus->devices); i++) {
>> +        devices = bus->devices[i];
>> +        if (!devices) {
>> +            continue;
>> +        }
>> +
>> +        /* skip itself */
>
> itself?

Same here, the comment is probably useless, the next line should be
easy to understand.

>> +        if (devices->devfn == dev->devfn) {
>> +            continue;
>> +        }
>> +
>> +        for (j = 0; j < PCI_NUM_REGIONS; j++) {
>
> This ignores bridges.

I'm not familiar with brigdes. I'll try to take a look.

>> +            r = &devices->io_regions[j];
>> +
>> +            /* skip different resource type, but don't skip when
>> +             * prefetch and non-prefetch memory are compared.
>> +             */
>> +            if (type != r->type) {
>> +                if (type & PCI_BASE_ADDRESS_SPACE_IO ||
>> +                    r->type & PCI_BASE_ADDRESS_SPACE_IO) {
>
> Do you mean
>        type & PCI_BASE_ADDRESS_SPACE_IO != r->type &
> PCI_BASE_ADDRESS_SPACE_IO
> ?
>
> This would not need a comment then.

Yes, I'll replace that.

>
>> +                    continue;
>> +                }
>> +            }
>> +
>> +            if ((addr < (r->addr + r->size)) && ((addr + size) > r->addr)) {
>
> Can ranges_overlap be used?

Yes.

>> +                printf("Overlapped to device[%02x:%02x.%x][Region:%d]"
>> +                       "[Address:%"PRIx64"h][Size:%"PRIx64"h]\n",
>> +                       pci_bus_num(bus), PCI_SLOT(devices->devfn),
>> +                       PCI_FUNC(devices->devfn), j, r->addr, r->size);
>
>
> That's not how one should report errors.

A callback would be better?


Thanks,

-- 
Anthony PERARD