[Xen-devel] Re: [PATCH 5/7] mm: New XENMEM space, XENMAPSPACE_gmfn_range

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.11.11 at 12:35, Jean Guyader <jean.guyader@xxxxxxxxxxxxx> wrote:
>@@ -4716,6 +4748,17 @@ long arch_memory_op(int op, XEN_GUEST_HANDLE(void) arg)
>         }
> 
>         rc = xenmem_add_to_physmap(d, &xatp);
>+        if ( rc == -EAGAIN )

        if ( rc )

>+        {
>+            if ( copy_to_guest(arg, &xatp, 1) )
>+            {
>+                rcu_unlock_domain(d);
>+                return -EFAULT;
>+            }

        }
        if ( rc == -EAGAIN )

(with some room for further simplification). Without that (or the minimal
alternative of copying back just .size or yet some other mechanism), as
pointed out before, the caller won't have a way to know how far into
the batch things succeeded.

>+
>+            rc = hypercall_create_continuation(
>+                    __HYPERVISOR_memory_op, "ih", op, arg);
>+        }
> 
>         rcu_unlock_domain(d);

Also, the whole block above can be move past this rcu_unlock_domain(),
eliminating the need to do it separately in the above error path(s).

> 
>--- a/xen/arch/x86/x86_64/compat/mm.c
>+++ b/xen/arch/x86/x86_64/compat/mm.c
>@@ -63,6 +63,18 @@ int compat_arch_memory_op(int op, XEN_GUEST_HANDLE(void) 
>arg)
> 
>         XLAT_add_to_physmap(nat, &cmp);
>         rc = arch_memory_op(op, guest_handle_from_ptr(nat, void));
>+        if ( rc < 0 )
>+            break;
>+

With the way the code below is currently this is superfluous. But just
as above you will need to provide some indication to the caller
*where* the failure occurred.

>+        if ( rc == __HYPERVISOR_memory_op )
>+        {
>+            hypercall_xlat_continuation(NULL, 0x2, nat, arg);
>+
>+            XLAT_add_to_physmap(&cmp, nat);
>+
>+            if ( copy_to_guest(arg, &cmp, 1) )
>+                return -EFAULT;

I realize that this is the same way in the code handling
XENMEM_[gs]et_pod_target, but unfortunately that's wrong (that's
why I'm copying you, George): Once a continuation was set up, you
mustn't change the return value anymore, since the continuation was
established by adjusting the guest's rIP.

As for other memory ops, the continuation can be encoded in "op" (see
xen/common/memory.c and xen/common/compat/memory.c). However,
while suitable here I don't think that's usable for the PoD variant. The
alternative is to cancel the continuation (would require quite a bit of
new code I think) or to adjust the low level hypercall handler code (at
least the compat mode one) to special case rAX values in the negative
errno range, leaving rAX unchanged instead of returning -ENOSYS.
Keir?

Jan

>+        }
> 
>         break;
>     }



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