On Mon, Sep 02, 2013 at 06:00:39PM +0800, Xiao Guangrong wrote: > On 09/02/2013 05:25 PM, Gleb Natapov wrote: > > On Mon, Sep 02, 2013 at 05:20:15PM +0800, Xiao Guangrong wrote: > >> On 08/30/2013 08:41 PM, Paolo Bonzini wrote: > >>> Page tables in a read-only memory slot will currently cause a triple > >>> fault because the page walker uses gfn_to_hva and it fails on such a slot. > >>> > >>> OVMF uses such a page table; however, real hardware seems to be fine with > >>> that as long as the accessed/dirty bits are set. Save whether the slot > >>> is readonly, and later check it when updating the accessed and dirty bits. > >> > >> Paolo, do you know why OVMF is using readonly memory like this? > >> > > Just a guess, but perhaps they want to move to paging mode as early as > > possible even before memory controller is fully initialized. > > > >> AFAIK, The fault trigged by this kind of access can hardly be fixed by > >> userspace since the fault is trigged by pagetable walking not by the current > >> instruction. Do you have any idea to let uerspace emulate it properly? > > Not sure what userspace you mean here, but there shouldn't be a fault in the > > I just wonder how to fix this kind of fault. The current patch returns -EACCES > but that will crash the guest. I think we'd better let userspace to fix this > error (let userspace set the D/A bit.) > Ugh, this is not good. Missed that. Don't know what real HW will do here, but the easy thing for us to do would be to just return success. > > first place if ROM page tables have access/dirty bit set and they do. > > Yes, so we can not call x86_emulate_instruction() to fix this fault (that function > emulates the access on the first place). Need directly return a MMIO-exit to > userpsace when met this fault? What happen if this fault on pagetable-walking > is trigged in x86_emulate_instruction().? I think we should not return MMIO-exit to userspace. Either ignore write attempt or kill a guest. -- Gleb. -- 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
