RE: [Xen-ia64-devel] PATCH: cleanup of tlbflush

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Tristan Gingold [mailto:Tristan.Gingold@xxxxxxxx]
>Sent: 2006年5月10日 18:47
>>
>> I see your concern about flush efficiency. However we still need set
>> necessary mask bits for correctness, right?
>Not yet, because pages are not transfered.

It's not specific to page flipping. Simple page sharing also has same 
problem.

>
>> It would be difficult to track
>> exact processors which have footprint about different ungranted
>pages.
>> To track that list may instead pull down performance at other places.
>> Then to set domain_dirty_cpumask as ones that domain is currently
>> running on, can be a simple/safe way in current stage though
>> performance may be affected.
>Unfortunatly performance are so badly affected that using SMP-g is
>useless!

If correctness becomes an issue, like shared va has footprint on 
several vcpus, you have to flush tlb on multiple processors or else 
SMP-g is broken.

After more thinking, I think there's no need for flush_tlb_mask to flush 
both tlb all and vhpt all. Flush_tlb_mask just does as what the name 
stands for: flushing all related TLBs indicating in the 
domain_dirty_cpumask. Instead the affected software structures can 
be always flushed in destroy_grant_host_mapping().

For xen/x86, destroy_grant_host_mapping clears affected pte entry in 
writable page table or the pte entry in shadow page table based on 
host_addr.

For xen/ia64, the vhpt table can be flushed by host_addr too, in 
destroy_grant_host_mapping. For each requested unmap page, only 
affected vhpt entry will be flushed and there's no need for full purge.

The key point is to pass in the gva address (host_addr) which is 
previously mapped to granted frame. It's guest's responsibility to record 
those mapped address and then passed in at unmap request. For 
example, xen/x86 use pre-allocated virtual address range while xen/ia64 
uses identity-mapped one. It's current para-driver style and we can
trust domain since guest needs to be cooperative or else domain itself 
is messed instead of xen.

Isaku, how about your thought on it?

Thanks,
Kevin

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