vpci: shrink critical section in vpci_{read/write}

Shrink critical section in vpci_{read/write} as racing calls to
vpci_{read,write}_hw() shouldn't be a problem. Those are just wrappers
around pci_conf_{read,write} functions, and the required locking (in
case of using the IO ports) is already taken care in pci_conf_{read,write}.

Please note, that we anyways split 64bit writes into two 32bit ones
without taking the lock for the whole duration of the access, so it is
possible to see a partially updated state as a result of a 64bit write:
the PCI(e) specification don't seem to specify whether the ECAM is allowed
to split memory transactions into multiple Configuration Requests and
whether those could then interleave with requests from a different CPU.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/drivers/vpci/vpci.c

@@ -370,6 +370,7 @@ uint32_t vpci_read(pci_sbdf_t sbdf, unsigned int reg, unsigned int size)
             break;
         ASSERT(data_offset < size);
     }
+    spin_unlock(&pdev->vpci->lock);
 
     if ( data_offset < size )
     {
@@ -379,7 +380,6 @@ uint32_t vpci_read(pci_sbdf_t sbdf, unsigned int reg, unsigned int size)
 
         data = merge_result(data, tmp_data, size - data_offset, data_offset);
     }
-    spin_unlock(&pdev->vpci->lock);
 
     return data & (0xffffffff >> (32 - 8 * size));
 }
@@ -475,13 +475,12 @@ void vpci_write(pci_sbdf_t sbdf, unsigned int reg, unsigned int size,
             break;
         ASSERT(data_offset < size);
     }
+    spin_unlock(&pdev->vpci->lock);
 
     if ( data_offset < size )
         /* Tailing gap, write the remaining. */
         vpci_write_hw(sbdf, reg + data_offset, size - data_offset,
                       data >> (data_offset * 8));
-
-    spin_unlock(&pdev->vpci->lock);
 }
 
 /* Helper function to check an access size and alignment on vpci space. */