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

[Cao jin]

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 <address@hidden>
> > ---
> > 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