Re: [Xen-devel] Re: [PATCH] let set_phys_to_machine return the old entry

[Stefan Berger <stefanb@xxxxxxxxxx>]

xen-devel-bounces@xxxxxxxxxxxxxxxxxxx wrote on 04/20/2006
03:08:08 PM:

> 
> On 20 Apr 2006, at 19:34, Stefan Berger wrote:
> 
> > The attached patch makes set_phys_to_machine(pfn,mfn) return
the old
> > entry in the array, such that the old entry can be restored again
using
> > the same call. If the feature XENFEAT_auto_translate_physmap
is not
> > enabled, then these two calls will end up in no-op relative to
changes
> > to the phys_to_machine_mapping array.
> >
> > There's currently only one occurrence where remembering the entry
from
> > the array is necessary and this is in the vTPM backend driver.
> >
> > This patch builds on top of the previously posted TPM backend
patch.
> 
> Why do you need to mess with the phys_to_machine table at all? You
> install the mapping only for a very short time, to do a memcpy or
> copy_from_user: neither of which will use that p2m entry. Even if
you 
> really do want to set the p2m entry properly you know that its old
> value must be INVALID_P2M_ENTRY because you obtained that kernel 
> virtual address via balloon_alloc_empty_page_range().
> 
> You should probably just delete the p2m code from that copy routine.

Going to do that and will repost.

  Stefan
> 
>   -- Keir
> 
> 
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