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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 22/08/14 19:28, Aravindh Puthiyaparambil (aravindp) wrote:
>> Please find below a POC patch that includes pausing and unpausing the
>> domain during the Xen writes to guest memory. I have it on top of the patch
>> that was using CR0.WP to highlight the difference. Please take a look and let
>> me know if this solution is acceptable.
>>> PS: I do realize whatever I do to create_bounce_frame() will have to be
>> reflected in the compat version. If this is correct approach I will do the same
>> there too.
>>> Thanks,
>>> Aravindh
>> I am concerned with the addition of a the vcpu specifics to
>> shadow_write_entries().  Most of the shadow code is already vcpu centric
>> where it should be domain centric, and steps are being made to alleviate
>> these problems.  
> All the call sites of shadow_write_entries() are vcpu specific which I why I thought it was OK to extend this to shadow_write_entries(). What are the steps being taken to alleviate the problems? Maybe I can piggy back on them?

You are not the first person to make this assumption.  I have a patch
series being worked on a "when I am not more busy" basis, but I don't
think there is anything useful you could piggy back on.

This problem aside, your current proposal does not work when crossing
page boundaries where the adjacent page is also read-only.  This is an
issue which really does need fixing.

Unfortunately, I am at a loss as to what to suggest.  No practical
solution comes to mind without using CR0.WP, and that has associated
problem.

~Andrew