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

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH] let set_phys_to_machine return the old entry
From: Stefan Berger <stefanb@xxxxxxxxxx>
Date: Thu, 20 Apr 2006 23:09:58 -0400
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
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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.

>   -- Keir
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