On 05/15/2015 10:04 AM, Andrew Jones wrote: > On Fri, May 15, 2015 at 08:02:59AM -0700, Christoffer Dall wrote: >> On Thu, May 14, 2015 at 03:32:13PM +0200, Andrew Jones wrote: >>> On Thu, May 14, 2015 at 12:55:49PM +0200, Christoffer Dall wrote: >>>> On Wed, May 13, 2015 at 01:31:54PM +0200, Andrew Jones wrote: >>>>> When S1 and S2 memory attributes combine wrt to caching policy, >>>>> non-cacheable types take precedence. If a guest maps a region as >>>>> device memory, which KVM userspace is using to emulate the device >>>>> using normal, cacheable memory, then we lose coherency. With >>>>> KVM_MEM_UNCACHED, KVM userspace can now hint to KVM which memory >>>>> regions are likely to be problematic. With this patch, as pages >>>>> of these types of regions are faulted into the guest, not only do >>>>> we flush the page's dcache, but we also change userspace's >>>>> mapping to NC in order to maintain coherency. >>>>> >>>>> What if the guest doesn't do what we expect? While we can't >>>>> force a guest to use cacheable memory, we can take advantage of >>>>> the non-cacheable precedence, and force it to use non-cacheable. >>>>> So, this patch also introduces PAGE_S2_NORMAL_NC, and uses it on >>>>> KVM_MEM_UNCACHED regions to force them to NC. >>>>> >>>>> We now have both guest and userspace on the same page (pun intended) >>>> >>>> I'd like to revisit the overall approach here. Is doing non-cached >>>> accesses in both the guest and host really the right thing to do here? >>> >>> I think so, but all ideas/approaches are still on the table. This is >>> still an RFC. >>> >>>> >>>> The semantics of the device becomes that it is cache coherent (because >>>> QEMU is), and I think Marc argued that Linux/UEFI should simply be >>>> adapted to handle whatever emulated devices we have as coherent. I also >>>> remember someone arguing that would be wrong (Peter?). >>> >>> I'm not really for quirking all devices in all guest types (AAVMF, Linux, >>> other bootloaders, other OSes). Windows is unlikely to apply any quirks. >>> >> >> Well my point was that if we're emulating a platform with coherent IO >> memory for PCI devices that is something that the guest should work with >> as such, but as Paolo explained it should always be safe for a guest to >> assume non-coherent, so that doesn't work. >> >>>> >>>> Finally, does this address all cache coherency issues with emulated >>>> devices? Some VOS guys had seen things still not working with this >>>> approach, unsure why... I'd like to avoid us merging this only to merge >>>> a more complete solution in a few weeks which reverts this solution... >>> >>> I'm not sure (this is still an RFT too :-) We definitely would need to >>> scatter some more memory_region_set_uncached() calls around QEMU first. >>> >> >> It would be good if you could sync with the VOS guys and make sure your >> patch set addresses their issues with the appropriate >> memory_region_set_uncached() added to QEMU, and if it does not, some >> vague idea why that falls outside of the scope of this patch set. After >> all, adding a USB controller to a VM is not that an esoteric use case, >> is it? > > I'll pull together a new version addressing all your comments, and also > put some more time into making sure it'll work... > > Jeremy, can you give me the qemu command line you're using for your tests? > I'll do some experimenting with it. > > Thanks, > drew > Hi Drew, I just recently looked at these patches and little confused. Where or how are the QEMU page tables changed to non-cacheable? I noticed the logical pfn is changed to non-cacheable. - Mario _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
