Re: [Xen-devel] [PATCH RFC v2 1/4] x86/mm: Shadow and p2m changes for PV mem_access

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.08.14 at 20:08, <aravindp@xxxxxxxxx> wrote:
>>>>> On 30.07.14 at 20:35, <aravindp@xxxxxxxxx> wrote:
>>>>Still rather ugly, and still leaving the window in time as big as the
>>>>CR0.CD variant (the window in space got shrunk to just a page).
>>>>Furthermore, how would you guarantee no other vCPU of the same guest
>>>>modifies the now writable page in the meantime?
>>>
>>> Good point. The only other thing I can think of to get around this
>>> issue is to pause the domain during these writes and unpause it on the
>>> way out of the guest access functions.
>>
>>If that's tolerable guest-performance-wise...
> 
> It would be for the use cases we. The performance hit would only be for 
> users who want to watch for writes to memory that is shared between Xen and 
> the guest. Given that which variant would you prefer between the CR0.WP and 
> pagetable methods?

Since the page table one has the overall smaller window of reduced
protection, I think I'd prefer that one. However, judging the overhead
acceptability by just the specific use case you have is perhaps
insufficient for including your changes in the public tree, especially
with no clear perspective of how to reduce it if someone indeed cared.

Jan


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