Re: [Xen-devel] [PATCH] hw/passthrough: Prevent QEMU from mapping PCI option ROM at address 0

[Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>]

On 12/05/14 14:09, Jan Beulich wrote:
>>>> On 12.05.14 at 14:42, <malcolm.crossley@xxxxxxxxxx> wrote:
>> The PCI option ROM BAR uses the LSB to indicate if the BAR is enabled.
>> The AMD graphics driver sets the address bit's of the BAR to 0 but leaves the
>> LSB set to 1. Whilst this is not good practice, QEMU should be ignoring the
>> non address parts of the BAR.
>>
>> This patch adds masking of the non address parts of the BAR before comparing
>> the address to 0.
>> ---
>>  hw/pass-through.c |    2 +-
>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/hw/pass-through.c b/hw/pass-through.c
>> index 304c438..7d6aefc 100644
>> --- a/hw/pass-through.c
>> +++ b/hw/pass-through.c
>> @@ -2208,7 +2208,7 @@ static void pt_bar_mapping_one(struct pt_dev *ptdev, 
>> int bar, int io_enable,
>>      }
>>  
>>      /* prevent guest software mapping memory resource to 00000000h */
>> -    if ((base->bar_flag == PT_BAR_FLAG_MEM) && (r_addr == 0))
>> +    if ((base->bar_flag == PT_BAR_FLAG_MEM) && ((r_addr & 
>> PCI_BASE_ADDRESS_MEM_MASK) == 0))
> 
> You talk of the low bit, but mask off the low 4 - how does that fit
> together? Didn't you rather mean PCI_ROM_ADDRESS_MASK &
> ~PCI_ROM_ADDRESS_ENABLE in text and code?

The description provides an example of a driver setting the lower bits
of the BAR.

The intent of the fix is to ensure no BAR is mapped address 0 which is
achieved by ensuring only the address bits of the BAR are used for the
comparison with 0.

Malcolm

> Jan
> 


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