[ adding Haozhong and Xiao for the alignment concerns below ] On Mon, Mar 7, 2016 at 10:58 AM, Toshi Kani <toshi.kani@xxxxxxx> wrote: > On Mon, 2016-03-07 at 09:18 -0800, Dan Williams wrote: >> On Mon, Mar 7, 2016 at 9:56 AM, Toshi Kani <toshi.kani@xxxxxxx> wrote: >> > On Fri, 2016-03-04 at 18:23 -0800, Dan Williams wrote: >> > > On Fri, Mar 4, 2016 at 6:48 PM, Toshi Kani <toshi.kani@xxxxxxx> >> > > wrote: >> [..] >> > > As far as I can see >> > > all we do is ask firmware implementations to respect Linux section >> > > boundaries and otherwise not change alignments. >> > >> > In addition to the requirement that pmem range alignment may not >> > change, the code also requires a regular memory range does not change >> > to intersect with a pmem section later. This seems fragile to me since >> > guest config may vary / change as I mentioned above. >> > >> > So, shouldn't the driver fails to attach when the range is not aligned >> > by the section size? Since we need to place a requirement to firmware >> > anyway, we can simply state that it must be aligned by 128MiB (at >> > least) on x86. Then, memory and pmem physical layouts can be changed >> > as long as this requirement is met. >> >> We can state that it must be aligned, but without a hard specification >> I don't see how we can guarantee it. We will fail the driver load >> with a warning if our alignment fixups end up getting invalidated by a >> later configuration change, but in the meantime we cover the gap of a >> BIOS that has generated a problematic configuration. > > I do not think it has to be stated in the spec (although it may be a good > idea to state it as an implementation note :-). > > This is an OS-unique requirement (and the size is x86-specific) that if it > wants to support Linux pmem pfn, then the alignment needs to be at least > 128MiB. Regular pmem does not have this restriction, but it needs to be > aligned by 2MiB or 1GiB for using huge page mapping, which does not have to > be stated in the spec, either. > > For KVM to support the pmem pfn feature on x86, it needs to guarantee this > 128MiB alignment. Otherwise, this feature is not supported. (I do not > worry about NVDIMM-N since it is naturally aligned by its size.) > > If we allow unaligned cases, then the driver needs to detect change from > the initial condition and fail to attach for protecting data. I did not > see such check in the code, but I may have overlooked. We cannot check if > KVM has any guarantee to keep the alignment at the initial setup, though. > > Thanks, > -Toshi -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>