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

[Jean Guyader <jean.guyader@xxxxxxxxxxxxx>]

On 11/11 09:21, Jan Beulich wrote:
> >>> On 11.11.11 at 10:13, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > On Fri, 2011-11-11 at 08:09 +0000, Jan Beulich wrote:
> >> >>> On 10.11.11 at 18:37, Jean Guyader <jean.guyader@xxxxxxxxxxxxx> wrote:
> >> > On 10/11 12:47, Jan Beulich wrote:
> >> >> >>> 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.
> >> >> 
> >> > 
> >> > In xenmem_add_to_physmap I modify xatp in place so when I exit this
> >> > function xatp will contain the updated value (new start value in
> >> > .gpfn and .idx, how far do I need to go in .size).
> >> > 
> >> > The idea here was to call the copy_to_guest only when we got preempted.
> >> > If I copy xatp back to the guest I should get the updated value
> >> > in xatp from the copy_from_guest when I'll be called by the continuation.
> >> 
> >> I understand the continuation aspect. But you appear to have not read
> >> my comments completely: I'm asking how your caller, in the event of
> >> failure, would know how much of the batch was processed successfully.
> > 
> > For this sort of flush operation can the caller assume that failure
> > means nothing was flushed, since a flush can always be repeated?
> 
> This is not just a flush - instead, the flush is just a necessary sub-
> operation of what is being done here. I don't think the actual add-
> to-physmap should be repeated. And even if it can, at least for
> diagnostic/debugging purposes knowing where things failed is rather
> useful.
> 

Ok, I'll do the copy_to_guest if rc isn't 0. Do you think I should do it
for all the spaces or only for gmfn_range?

Jean

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel