Re: [PATCH v3 kvmtool 25/32] vfio/pci: Don't write configuration value twice

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On 4/2/20 9:35 AM, André Przywara wrote:
> On 26/03/2020 15:24, Alexandru Elisei wrote:
>> After writing to the device fd as part of the PCI configuration space
>> emulation, we read back from the device to make sure that the write
>> finished. The value is read back into the PCI configuration space and
>> afterwards, the same value is copied by the PCI emulation code. Let's
>> read from the device fd into a temporary variable, to prevent this
>> double write.
>>
>> The double write is harmless in itself. But when we implement
>> reassignable BARs, we need to keep track of the old BAR value, and the
>> VFIO code is overwritting it.
>>
> It seems still a bit fragile, since we rely on code in other places to
> limit "sz" to 4 or less, but in practice we should be covered.
>
> Can you maybe add an assert here to prevent accidents on the stack?

Sure, sounds reasonable.

Thanks,
Alex
>
>> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> Cheers,
> Andre
>
>> ---
>>  vfio/pci.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/vfio/pci.c b/vfio/pci.c
>> index fe02574390f6..8b2a0c8dbac3 100644
>> --- a/vfio/pci.c
>> +++ b/vfio/pci.c
>> @@ -470,7 +470,7 @@ static void vfio_pci_cfg_write(struct kvm *kvm, struct pci_device_header *pci_hd
>>  	struct vfio_region_info *info;
>>  	struct vfio_pci_device *pdev;
>>  	struct vfio_device *vdev;
>> -	void *base = pci_hdr;
>> +	u32 tmp;
>>  
>>  	if (offset == PCI_ROM_ADDRESS)
>>  		return;
>> @@ -490,7 +490,7 @@ static void vfio_pci_cfg_write(struct kvm *kvm, struct pci_device_header *pci_hd
>>  	if (pdev->irq_modes & VFIO_PCI_IRQ_MODE_MSI)
>>  		vfio_pci_msi_cap_write(kvm, vdev, offset, data, sz);
>>  
>> -	if (pread(vdev->fd, base + offset, sz, info->offset + offset) != sz)
>> +	if (pread(vdev->fd, &tmp, sz, info->offset + offset) != sz)
>>  		vfio_dev_warn(vdev, "Failed to read %d bytes from Configuration Space at 0x%x",
>>  			      sz, offset);
>>  }
>>