Re: [Qemu-devel] [PATCH] bugfix: passing reference instead of value

[Cao jin <caoj.fnst@cn.fujitsu.com>]

On 12/28/2015 07:50 PM, Michael S. Tsirkin wrote:
> On Mon, Dec 28, 2015 at 10:54:23AM +0800, Cao jin wrote:
>> Fix the bug introduced by 595a4f07. Function host_pci_config_read() should be
>> passed by a reference, not a value, for the later pci_default_write_config().
>
> What's the effect of the bug? Does it break igd assignment?
> How come it worked for people?
> If the function is never called, mayber we can get rid
> of it completely?
>

sorry if I didn`t explain it clearly to you. let me try the explanation 
again: This function is called only when using 
TYPE_IGD_PASSTHROUGH_I440FX_PCI_DEVICE(when realize it)

the effect of the bug:
pci_default_write_config(pci_dev, pos, val, len);
                                         ^ *its value is always 0*

I think it won`t break igd assignment, but just give a wrong register 
value(forever 0) in PCI config space(wrong register value may results in 
abnormal working state?). the register should get its value by 
host_pci_config_read()

Because my bad English description, Let me do a analogy, here is the 
imitation of original code:

void swap(unsigned int val)  //this is host_pci_config_read()
{
     unsigned int org = 2;
     memcpy(&val, &org, sizeof(unsigned int));
}

int main()
{
     unsigned int val = 0;

     swap(val);

     printf("val = %d\n", val);
     return 0;
}

author want to get: val = 2. but it will always: val = 0; This is 
exactly the bug I find.

> Stefano?
>
>>
>> Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
>> ---
>> Separated from previous "igd-passthru convert to realize" patch. Since these
>> two don`t have dependency, can send it solely.
>>
>> Not test since it is easy to find out if reading carefully, just compiled.
>>
>>   hw/pci-host/piix.c | 8 +++++---
>>   1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/hw/pci-host/piix.c b/hw/pci-host/piix.c
>> index 715208b..924f0fa 100644
>> --- a/hw/pci-host/piix.c
>> +++ b/hw/pci-host/piix.c
>> @@ -761,7 +761,7 @@ static const IGDHostInfo igd_host_bridge_infos[] = {
>>       {0xa8, 4},  /* SNB: base of GTT stolen memory */
>>   };
>>
>> -static int host_pci_config_read(int pos, int len, uint32_t val)
>> +static int host_pci_config_read(int pos, int len, uint32_t *val)
>>   {
>>       char path[PATH_MAX];
>>       int config_fd;
>> @@ -784,12 +784,14 @@ static int host_pci_config_read(int pos, int len, uint32_t val)
>>           ret = -errno;
>>           goto out;
>>       }
>> +
>>       do {
>> -        rc = read(config_fd, (uint8_t *)&val, len);
>> +        rc = read(config_fd, (uint8_t *)val, len);
>>       } while (rc < 0 && (errno == EINTR || errno == EAGAIN));
>>       if (rc != len) {
>>           ret = -errno;
>>       }
>> +
>>   out:
>>       close(config_fd);
>>       return ret;
>> @@ -805,7 +807,7 @@ static int igd_pt_i440fx_initfn(struct PCIDevice *pci_dev)
>>       for (i = 0; i < num; i++) {
>>           pos = igd_host_bridge_infos[i].offset;
>>           len = igd_host_bridge_infos[i].len;
>> -        rc = host_pci_config_read(pos, len, val);
>> +        rc = host_pci_config_read(pos, len, &val);
>>           if (rc) {
>>               return -ENODEV;
>>           }
>> --
>> 2.1.0
>>
>>
>
>
> .
>

-- 
Yours Sincerely,

Cao Jin