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

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

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.

 -- Keir


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