Hi Avi, On 08/03/2011 04:09 PM, Avi Kivity wrote: > On 08/03/2011 09:02 AM, Xiao Guangrong wrote: >> > Note we don't need to actually emulate, just decode, since page_fault can tell us whether a write failed due to page tables or mmio. >> > >> >> This is a interesting feature. If it happens, i will just drop the shadow pages >> and retry these instructions directly. > > Note it's a little dangerous. If the guest uses a non-page-table modifying instruction on the PDE that points to the instruction, then we will unmap the instruction and go to an infinite loop. > Yes, it is. > Maybe it's better to emulate if we can't find a fix for that. > > One way would be to emulate every 20 instructions; this breaks us out of the loop but reduces costly emulations to 5%. > After much thought about this, may be this optimization is not good since: - it is little complex - this optimization is only applied to the instruction emulation caused by #PF - it does not improve too much: if we emulate the instruction, we need to do: - decode instruction - emulate it - zap shadow pages And do this, it can return to the guest, the guest can run the next instruction if we retry the instruction, we need to do: - decode instruction - zap shadow pages then return to the guest and retry the instruction, however, we will get page fault again(since the mapping is still read-only), so we will get another VM-exit and need to do: # trigger page fault - handle the page fault and change the mapping to writable - retry the instruction until now, the guest can run the next instruction So, i do not think the new way is better, your opinion？ -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
