Re: [PATCH v8 06/15] ARM: KVM: Hypervisor identity mapping

[Avi Kivity <avi@redhat.com>]

On 06/18/2012 08:55 PM, Christoffer Dall wrote:
> On Mon, Jun 18, 2012 at 9:12 AM, Avi Kivity <avi@redhat.com> wrote:
>> On 06/15/2012 10:07 PM, Christoffer Dall wrote:
>>> Adds support in the identity mapping feature that allows KVM to setup
>>> identity mapping for the Hyp mode with the AP[1] bit set as required by
>>> the specification and also supports freeing created sub pmd's after
>>> finished use.
>>>
>>> These two functions:
>>>  - hyp_idmap_add(pgd, addr, end);
>>>  - hyp_idmap_del(pgd, addr, end);
>>> are essentially calls to the same function as the non-hyp versions but
>>> with a different argument value. KVM calls these functions to setup
>>> and teardown the identity mapping used to initialize the hypervisor.
>>>
>>> Note, the hyp-version of the _del function actually frees the pmd's
>>> pointed to by the pgd as opposed to the non-hyp version which just
>>> clears them.
>>
>>
>> I asked previously what happens if two data structures share a page, and
>> one of them is removed.  Is that handled now?  How?
>>
> 
> I think you asked previously for the general hyp-mode mappings, not
> the identity mappings. For the general hyp-mode mappings we simply
> don't unmap the data structures, potentially leaking a few pages for
> the page tables themselves.
> 
> This is only for initialization, so there are not really any data
> structures mapped, only one/two pages to initialize the hypervisor
> mode.
> 
>> Why not just identity map all memory?  You can use large pages so it's
>> fast and doesn't consume a lot of page table memory.
> 
> That's an option, but it still seems like an awful waste since it's
> only used once (unless you unload and re-load the module) and there's
> really no problem with data structures here.
> 
> The truth is that this is going to go away, and the code will be put
> in a section that's idmapped from kernel start. There's a patch under
> way from Marc taking care of this which I assmue we'll merge for v9.

Okay, thanks.


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