Re: [Qemu-devel] [PATCH] pci: do not pci_update_mappings when guest gets bar size

[Gonglei <arei.gonglei@huawei.com>]

On 2014/10/14 20:15, chenliang (T) wrote:

> On 2014/10/14 19:48, Michael S. Tsirkin wrote:
> 
>> On Tue, Oct 14, 2014 at 07:41:14PM +0800, ChenLiang wrote:
>>> We find overlap when the size of pci bar is bigger then 16MB, it overlaps with private
>>> memslot in the kmod. By the way, the new kmod skip private memslot. But I think if the size
>>> of pci bar is enough big, it also  overlaps with other memslots.
>>
>> Of course but it should not cause a crash.
>> If you need the overlapping memslot available during the programming
>> process, increase it's priority.
>>
> 
> Yeah, I know the priority of memory region.
> The problem is overlaping should not happen when one pci bar is not
> overlap with any other memslots. But Qemu always do pci_update_mappings
> when guest os writes pci bar. Actually, should not do pci_update_mappings
> if var is 0xffffffff.
> 

The PCI spec says [Page 222]:
Decode (I/O or memory) of a register is disabled via the command register before sizing a
Base Address register. Software saves the original value of the Base Address register,
writes 0FFFFFFFFh to the register, then reads it back.

>>> the root cause is:
>>>
>>> pci_default_write_config will do that:
>>>     for (i = 0; i < l; val >>= 8, ++i) {
>>>         uint8_t wmask = d->wmask[addr + i];
>>>         uint8_t w1cmask = d->w1cmask[addr + i];
>>>         assert(!(wmask & w1cmask));
>>>         d->config[addr + i] = (d->config[addr + i] & ~wmask) | (val & wmask);
>>>         d->config[addr + i] &= ~(val & w1cmask); /* W1C: Write 1 to Clear */
>>>     }
>>>
>>> *(int*)(d->config[addr]) will be 0xfe00000c, if val is 0xffffffff and the size of bar is 32MB.
>>> This range overlap with private memslot in the old kmod.
>>>
>>> then pci_update_mappings will update memslot.
>>>
>>> On 2014/10/14 19:20, Michael S. Tsirkin wrote:
>>>
>>>> On Tue, Oct 14, 2014 at 07:04:14PM +0800, arei.gonglei@huawei.com wrote:
>>>>> From: ChenLiang <chenliang88@huawei.com>
>>>>>
>>>>> Power-up software can determine how much address space the device
>>>>> requires by writing a value of all 1's to the register and then
>>>>> reading the value back(PCI specification). Qemu should not do
>>>>> pci_update_mappings. Qemu may exit, because the wrong address of
>>>>> this bar is overlap with other memslots.
>>>>>
>>>>> Signed-off-by: ChenLiang <chenliang88@huawei.com>
>>>>> Signed-off-by: Gonglei <arei.gonglei@huawei.com>
>>>>
>>>> This is at best a work-around.
>>>> Overlapping is observed in practice, qemu really shouldn't exit when
>>>> this happens.
>>>> So we should find the root cause and fix it there instead of
>>>> adding work-arounds in PCI core.
>>>>
>>>> With which device do you observe this?

ivshmem device with 32M' bar2 size.

Best regards,
-Gonglei

>>>>
>>>>
>>>>> ---
>>>>>  hw/pci/pci.c | 8 ++++----
>>>>>  1 file changed, 4 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/hw/pci/pci.c b/hw/pci/pci.c
>>>>> index 6ce75aa..4d44b44 100644
>>>>> --- a/hw/pci/pci.c
>>>>> +++ b/hw/pci/pci.c
>>>>> @@ -1158,12 +1158,12 @@ void pci_default_write_config(PCIDevice *d, uint32_t addr, uint32_t val_in, int
>>>>>          d->config[addr + i] = (d->config[addr + i] & ~wmask) | (val & wmask);
>>>>>          d->config[addr + i] &= ~(val & w1cmask); /* W1C: Write 1 to Clear */
>>>>>      }
>>>>> -    if (ranges_overlap(addr, l, PCI_BASE_ADDRESS_0, 24) ||
>>>>> +    if (((ranges_overlap(addr, l, PCI_BASE_ADDRESS_0, 24) ||
>>>>>          ranges_overlap(addr, l, PCI_ROM_ADDRESS, 4) ||
>>>>> -        ranges_overlap(addr, l, PCI_ROM_ADDRESS1, 4) ||
>>>>> -        range_covers_byte(addr, l, PCI_COMMAND))
>>>>> +        ranges_overlap(addr, l, PCI_ROM_ADDRESS1, 4)) &&
>>>>> +        val_in != 0xffffffff) || range_covers_byte(addr, l, PCI_COMMAND)) {
>>>>>          pci_update_mappings(d);
>>>>> -
>>>>> +    }
>>>>>      if (range_covers_byte(addr, l, PCI_COMMAND)) {
>>>>>          pci_update_irq_disabled(d, was_irq_disabled);
>>>>>          memory_region_set_enabled(&d->bus_master_enable_region,
>>>>> -- 
>>>>> 1.7.12.4
>>>>>
>>>>
>>>> .
>>>>
>>>
>>>
>>
>> .
>>
> 
> 
>