On Mon, Dec 14, 2015 at 08:34:00AM -0800, Alexander Duyck wrote: > > This way distro can use a guest agent to disable > > dirtying until before migration starts. > > Right. For a v2 version I would definitely want to have some way to > limit the scope of this. My main reason for putting this out here is > to start altering the course of discussions since it seems like were > weren't getting anywhere with the ixgbevf migration changes that were > being proposed. Absolutely, thanks for working on this. > >> + unsigned long pg_addr, start; > >> + > >> + start = (unsigned long)addr; > >> + pg_addr = PAGE_ALIGN(start + size); > >> + start &= ~(sizeof(atomic_t) - 1); > >> + > >> + /* trigger a write fault on each page, excluding first page */ > >> + while ((pg_addr -= PAGE_SIZE) > start) > >> + atomic_add(0, (atomic_t *)pg_addr); > >> + > >> + /* trigger a write fault on first word of DMA */ > >> + atomic_add(0, (atomic_t *)start); > > > > start might not be aligned correctly for a cast to atomic_t. > > It's harmless to do this for any memory, so I think you should > > just do this for 1st byte of all pages including the first one. > > You may not have noticed it but I actually aligned start in the line > after pg_addr. Yes you did. alignof would make it a bit more noticeable. > However instead of aligning to the start of the next > atomic_t I just masked off the lower bits so that we start at the > DWORD that contains the first byte of the starting address. The > assumption here is that I cannot trigger any sort of fault since if I > have access to a given byte within a DWORD I will have access to the > entire DWORD. I'm curious where does this come from. Isn't it true that access is controlled at page granularity normally, so you can touch beginning of page just as well? > I coded this up so that the spots where we touch the > memory should match up with addresses provided by the hardware to > perform the DMA over the PCI bus. Yes but there's no requirement to do it like this from virt POV. You just need to touch each page. > Also I intentionally ran from highest address to lowest since that way > we don't risk pushing the first cache line of the DMA buffer out of > the L1 cache due to the PAGE_SIZE stride. > > - Alex Interesting. How does order of access help with this? By the way, if you are into these micro-optimizations you might want to limit prefetch, to this end you want to access the last line of the page. And it's probably worth benchmarking a bit and not doing it all just based on theory, keep code simple in v1 otherwise. -- MST -- 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
