Re: [PATCH 0/4] Swapping

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Avi Kivity wrote:
> Anthony Liguori wrote:
>> Izik Eidus wrote:
>>  
>>>> That's not quite what I was wondering.
>>>>
>>>> When you do an madvise() in userspace, the result is that when that 
>>>> memory is accessed again, linux will demand-fault in a zero page 
>>>> and COW it appropriately.  If we do madvise() on the VA 
>>>> representing guest physical memory, what I'm curious about is 
>>>> whether the guest will actually see this change.  If the guest 
>>>> happens to have the page mapped before we do the madvise(), what 
>>>> triggers KVM to kick any shadow page table entries out of it's cache?
>>>>
>>>> IIUC, today, after the madvise, the guest will have access to the 
>>>> old page until that entry gets evicted and reloaded from the shadow 
>>>> page table cache.
>>>>       
>>> ok i am no familier with madvise() so i might talk nonsense but,
>>> if the guest have the page mapped before the madvise(), this mean we 
>>> have high refernce to it, this is our only protection and as far as 
>>> i understand this should be enough
>>>     
>>
>> Right, we will have a reference to the page.  But we want to 
>> propagate this change to the guest.   So madvise() may be a bad example.
>>
>> What if you wanted to do shared memory for multiple guests.  You 
>> start out with an anonymous mmap(), and you now what to mmap() a file 
>> in /dev/shm to be shared among multiple guests so you mmap(MAP_FIXED) 
>> to phys_ram_base + guest_pa in each guest.
>>
>> So what if guest_pa was in the guest's shadow page cache?  In order 
>> for the guest to see the right hpa (the new shared memory), we have 
>> to be able to evict guest_pa.  We could do this with something like 
>> mmu_unshadow().
>>
>> What I don't understand, is how we can have something like 
>> mmu_unshadow() called automatically when an mmap() is initiated from 
>> userspace.  We could just add an ioctl() to do it from userspace but 
>> I think it would be nicer if it Just Worked.
>>   
>
> Behold the magic of pte notifiers!  Every time the host touches a host 
> page table entry, it calls kvm which zaps the corresponding shadow pte 
> entries and invalidates any tlb entries in running vcpus.

/me bows down to the greatness of pte notifiers

So yeah, that would solve the problem nicely.  Are you planning on 
resubmitting those patches or did they end up in Linus' tree?

Regards,

Anthony Liguori

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