Re: [Xen-devel] [PATCH RFC 4/6] xen: Print and use errno where applicable.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Jul 01, 2015 at 02:01:07PM +0100, Stefano Stabellini wrote:
> On Mon, 29 Jun 2015, Konrad Rzeszutek Wilk wrote:
> > In Xen 4.6 commit cd2f100f0f61b3f333d52d1737dd73f02daee592
> > "libxc: Fix do_memory_op to return negative value on errors"
> > made the libxc API less odd-ball: On errors, return value is
> > -1 and error code is in errno. On success the return value
> > is either 0 or an positive value.
> > 
> > Since we could be running with an old toolstack in which the
> > Exx value is in rc or the newer, we print both and return
> > the -EXX depending on rc == -1 condition.
> > 
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > ---
> >  xen-hvm.c | 10 ++++++----
> >  1 file changed, 6 insertions(+), 4 deletions(-)
> > 
> > diff --git a/xen-hvm.c b/xen-hvm.c
> > index 0408462..a92bc14 100644
> > --- a/xen-hvm.c
> > +++ b/xen-hvm.c
> > @@ -345,11 +345,12 @@ go_physmap:
> >          unsigned long idx = pfn + i;
> >          xen_pfn_t gpfn = start_gpfn + i;
> >  
> > +        /* In Xen 4.6 rc is -1 and errno contains the error value. */
> >          rc = xc_domain_add_to_physmap(xen_xc, xen_domid, XENMAPSPACE_gmfn, 
> > idx, gpfn);
> >          if (rc) {
> >              DPRINTF("add_to_physmap MFN %"PRI_xen_pfn" to PFN %"
> > -                    PRI_xen_pfn" failed: %d\n", idx, gpfn, rc);
> > -            return -rc;
> > +                    PRI_xen_pfn" failed: %d (errno: %d)\n", idx, gpfn, rc, 
> > errno);
> > +            return rc == -1 ? -errno : -rc;
> 
> Printing both rc and errno is the right thing to do, but I am not sure
> changing return value depending on the libxc version is a good idea.
> Maybe we should be consistent and always return rc?

In Xen 4.5 and earlier this function would return -EINVAL (say rc=EINVAL).
With Xen 4.6 it would always return 1 on errors (rc is -1, and with --1 we get 
1), while
the errno would have EINVAL.

To be consistent and have this function return an proper -Exx value we
need that check to use errno in case rc == -1.

I am uncomfortable with returning positive values as errors, which reminds me -
I need to update the commit to mention the return 1 issue.
> 
> 
> >          }
> >      }
> >  
> > @@ -422,11 +423,12 @@ static int xen_remove_from_physmap(XenIOState *state,
> >          xen_pfn_t idx = start_addr + i;
> >          xen_pfn_t gpfn = phys_offset + i;
> >  
> > +        /* In Xen 4.6 rc is -1 and errno contains the error value. */
> >          rc = xc_domain_add_to_physmap(xen_xc, xen_domid, XENMAPSPACE_gmfn, 
> > idx, gpfn);
> >          if (rc) {
> >              fprintf(stderr, "add_to_physmap MFN %"PRI_xen_pfn" to PFN %"
> > -                    PRI_xen_pfn" failed: %d\n", idx, gpfn, rc);
> > -            return -rc;
> > +                    PRI_xen_pfn" failed: %d (errno: %d)\n", idx, gpfn, rc, 
> > errno);
> > +            return rc == -1 ? -errno : -rc;
> >          }
> >      }
> >  
> > -- 
> > 2.1.0
> > 

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