On 15.08.2012, at 19:47, Scott Wood wrote: > On 08/15/2012 12:27 PM, Alexander Graf wrote: >> >> On 15.08.2012, at 19:26, Scott Wood wrote: >> >>> On 08/15/2012 04:52 AM, Alexander Graf wrote: >>>> >>>> On 15.08.2012, at 03:23, Scott Wood wrote: >>>> >>>>> On 08/14/2012 06:04 PM, Alexander Graf wrote: >>>>>> When we map a page that wasn't icache cleared before, do so when first >>>>>> mapping it in KVM using the same information bits as the Linux mapping >>>>>> logic. That way we are 100% sure that any page we map does not have stale >>>>>> entries in the icache. >>>>> >>>>> We're not really 100% sure of that -- this only handles the case where >>>>> the kernel does the dirtying, not when it's done by QEMU or the guest. >>>> >>>> When the guest does it, the guest is responsible for clearing the >>>> icache. Same for QEMU. It needs to clear it when doing DMA. >>> >>> Sure. I was just worried that that commit message could be taken the >>> wrong way, as in "we no longer need the QEMU icache flushing patch". >>> >>>> However, what is still broken would be a direct /dev/mem map. There >>>> QEMU should probably clear the icache before starting the guest, in >>>> case another guest was running on that same memory before. >>>> Fortunately, we don't have that mode available in upstream QEMU :). >>> >>> How is QEMU loading images different if it's /dev/mem versus ordinary >>> anonymous memory? You probably won't have stale icache data in the >>> latter case (which makes it less likely to be a problem in pratice), but >>> in theory you could have data that still hasn't left the dcache. >> >> It's the same. I just talked to Ben about this today in a different context and we should be safe :). > > Safe how? > > If it's truly the same, we're definitely not safe, since I had problems > with this using /dev/mem (particularly when changing the kernel image > without a host reboot) before I put in the icache flush patch. QEMU needs to icache flush everything it puts into guest memory. Whatever blob comes next (SLOF for -M pseries, kernel for -M e500) assumes dirty icache for every page it maps itself. So SLOF will icache flush the kernel text segment, Linux will icache flush user space pages. Alex -- 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
