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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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?

Ian.

> 
> Jan
> 
> 
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