Re: [PATCH 0/4] Swapping

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

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.

Regards,

Anthony Liguori

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