Re: [Qemu-devel] Intel X86 hardware transactional memory

[Richard Henderson <rth@twiddle.net>]

On 02/04/2015 09:09 PM, Patrick Williams III wrote:
>>I think one possible way is to clear the software tlb on entry to the
>>transaction and disable (not install any translations) the software
>>TLB till the end of the transaction. 
>>In the softmmu helper functions, the memory addresses as well as
>>value can then be tracked in a hashtable and committed to system
>>memory once the transaction is ready to be committed.
> 
> I believe the transaction conflicts are handled post-translation by the cache
> coherency protocol.  Trying to store the pre-translation address isn't
> sufficient for conflict resolution.
> 
> Consider two virtual addresses that point to the same physical address.  One
> thread uses the first virtual address in a transaction; another thread writes
> to the second virtual address while the transaction is going on.  This should
> cause the transaction to fail because the same physical address was involved.

That's not really relevant.  The important part is that no TLB hits are made
within TCG, so that we always use the softmmu helpers.

Incidentally, that's also accomplished by bits such as TLB_NOTDIRTY.  We could
add another bit so that pages referenced by TM don't need to be fetched out of
the page tables every time.


r~