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Re: [Xen-devel] [PATCH] xen/x86: p2m: Don't initialize slot 0 of the P2M
- To: Julien Grall <julien@xxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: George Dunlap <george.dunlap@xxxxxxxxxx>
- Date: Mon, 3 Feb 2020 17:37:17 +0000
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- Cc: Wei Liu <wl@xxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Julien Grall <jgrall@xxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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On 2/3/20 5:22 PM, Julien Grall wrote:
> Hi,
>
> On 03/02/2020 17:10, George Dunlap wrote:
>> On 2/3/20 4:58 PM, Julien Grall wrote:
>>> From: Julien Grall <jgrall@xxxxxxxxxx>
>>>
>>> It is not entirely clear why the slot 0 of each p2m should be populated
>>> with empty page-tables. The commit introducing it 759af8e3800 "[HVM]
>>> Fix 64-bit HVM domain creation." does not contain meaningful
>>> explanation except that it was necessary for shadow.
>>
>> Tim, any ideas here?
>>
>>> As we don't seem to have a good explanation why this is there, drop the
>>> code completely.
>>>
>>> This was tested by successfully booting a HVM with shadow enabled.
>>>
>>> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
>>>
>>> ---
>>>
>>> I don't know for sure if this is going to break a setup. I have tried
>>> HVM guest with hap={0, 1} without any trouble. I am happy to try more
>>> setup if you have any in mind.
>>>
>>> If this break a setup, then please describe the setup and I will send a
>>> documentation patch instead.
>>
>> This is a somewhat risky strategy. Other than code clean-up, is there
>> any advantage to removing this code at the moment?
>
> If Tim doesn't have an explanation, then we have two solutions:
> 1) Checkin the code and see if that breaks
> 2) Keep code we have no clue why it is there
It is probably early enough in the dev cycle to do this.
Also, it's not clear to me what kind of bug the code you're deleting
would fix. If you read a not-present entry, you should get INVALID_MFN
anyway. Unless you were calling p2m_get_entry_query(), which I'm pretty
sure hadn't been introduced at this point.
> I understand that the former is risky, but the latter is not very ideal
> either because if we can't explain the reason now, then it is unlikely
> that we would in the future.
>
> Regarding the advantage of removing it, I am looking at liveupdate and
> how to keep the P2M around. I am trying to limit the number of "if
> (liveupdate)" within the code. So any cleanup would be beneficial.
OK, thanks.
-George
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