[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH 4/4] hvmloader: add support to load extra ACPI tables from qemu
>>> On 26.01.16 at 16:30, <haozhong.zhang@xxxxxxxxx> wrote: > On 01/26/16 05:44, Jan Beulich wrote: >> Interesting. This isn't the usage model I have been thinking about >> so far. Having just gone back to the original 0/4 mail, I'm afraid >> we're really left guessing, and you guessed differently than I did. >> My understanding of the intentions of PMEM so far was that this >> is a high-capacity, slower than DRAM but much faster than e.g. >> swapping to disk alternative to normal RAM. I.e. the persistent >> aspect of it wouldn't matter at all in this case (other than for PBLK, >> obviously). > > Of course, pmem could be used in the way you thought because of its > 'ram' aspect. But I think the more meaningful usage is from its > persistent aspect. For example, the implementation of some journal > file systems could store logs in pmem rather than the normal ram, so > that if a power failure happens before those in-memory logs are > completely written to the disk, there would still be chance to restore > them from pmem after next booting (rather than abandoning all of > them). Well, that leaves open how that file system would find its log after reboot, or how that log is protected from clobbering by another OS booted in between. >> However, thinking through your usage model I have problems >> seeing it work in a reasonable way even with virtualization left >> aside: To my knowledge there's no established protocol on how >> multiple parties (different versions of the same OS, or even >> completely different OSes) would arbitrate using such memory >> ranges. And even for a single OS it is, other than for disks (and >> hence PBLK), not immediately clear how it would communicate >> from one boot to another what information got stored where, >> or how it would react to some or all of this storage having >> disappeared (just like a disk which got removed, which - unless >> it held the boot partition - would normally have pretty little >> effect on the OS coming back up). > > Label storage area is a persistent area on NVDIMM and can be used to > store partitions information. It's not included in pmem (that part > that is mapped into the system address space). Instead, it can be only > accessed through NVDIMM _DSM method [1]. However, what contents are > stored and how they are interpreted are left to software. One way is > to follow NVDIMM Namespace Specification [2] to store an array of > labels that describe the start address (from the base 0 of pmem) and > the size of each partition, which is called as namespace. On Linux, > each namespace is exposed as a /dev/pmemXX device. According to what I've just read in one of the documents Konrad pointed us to, there can be just one PMEM label per DIMM. Unless I misread of course... Jan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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