Re: [Xen-devel] [PATCH v4] altp2m: Allow the hostp2m entries to be of type p2m_ram_shared

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 18/07/16 16:27, Andrew Cooper wrote:
> On 18/07/16 16:18, Tamas K Lengyel wrote:
>> On Mon, Jul 18, 2016 at 5:04 AM, George Dunlap <george.dunlap@xxxxxxxxxx> 
>> wrote:
>>> On Fri, Jul 15, 2016 at 3:59 PM, Tamas K Lengyel <tamas@xxxxxxxxxxxxx> 
>>> wrote:
>>>>> I could go on in the analysis, but the point is that there's a morass
>>>>> of interactions here all of which need to be correct, which this patch
>>>>> does not address.  You have a long way to go before sharing and altp2m
>>>>> can be safely used on the same gfn ranges.
>>>>>
>>>> Hi George,
>>>> certainly there are cornercases where if the user does not setup things in
>>>> the right order things can still go out of whack. My goal with this patch 
>>>> is
>>>> not to address all of them. The goal of this patch is to not crash the
>>>> hypervisor when the user at least tries to experiment with it, which is the
>>>> current state. So this patch improves the status quo. Also, both 
>>>> mem_sharing
>>>> and altp2m is considered experimental/tech_preview, so the fact that their
>>>> combination is also experimental should be assumed as well. As I explained,
>>>> with this patch in place there is at least one way they can be safely used
>>>> together if the user tracks unsharing requirements through mem_access and
>>>> does unsharing and fixup of the altp2m views manually. There are other ways
>>>> where it would not be safe as after unsharing we don't know how the user
>>>> would want things to look in altp2ms. I don't think we want to start
>>>> guessing about that either so I will not be looking to implement that. So I
>>>> don't agree with this reasoning being grounds for rejecting this patch that
>>>> does incrementally improve the current state.
>>> So you keep saying "user"; I assume you mean whatever program is
>>> sitting in domain 0, which is going to be doing memsharing, altp2m,
>>> and memaccess stuff all at once?
>>>
>>> The altp2m code was not written for that purpose.  It was written for
>>> *guest* administrators to use within the guest.
>> That's simply not true. It was written specifically to allow both
>> usecases - both internal *and* external. Mixing the use-cases was not
>> envisioned.
> 
> Tamas is correct here.  altp2m was specifically designed and implemented
> usable by both internal and external entities, irrespective of hardware
> support.
> 
> Any modifications to the subsystem should maintain this property.

Indeed; it was also designed to be robust when used with sharing
(although it was apparently never tested, because it's currently broken
in that respect).  And it is this property that I'm trying to maintain,
both for internal and external users.

 -George

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