Re: [Crash-utility] crash and libvirt, and more

Richard W.M. Jones wrote: It depends upon the architecture, but all of them have at least one kernel virtual address range where, the PAGE_OFFSET value can be stripped off, resulting in the physical address. So "unity-mapping" just means a one-to-one address translation, since the physical address is right "there" encoded in the virtual address value. For bare-metal kernels, the physical address is then read directly from the dumpfile or /dev/mem -- or since Red Hat restricts /dev/mem, the /dev/crash driver is loaded as replacement for /dev/mem. For xen kernels specifically, the stripped address is a pseudo-physical address, but the xendump dumpfile formats (the "old" and newer-ELF-style formats) require those to be translated to machine-address values. So then the p2m table needs to be accessed to turn the pseudo-physical into a machine address, and then that address is what is found and accessed from the dumpfile. On the other hand, vmalloc() addresses are a range of kernel virtual addresses, typically above PAGE_OFFSET (but not necessarily on all architectures), that are mapped to some unknown physical address. Therefore, they *do* require a full page-table walkthrough, because, like user virtual addresses, there's no indication in the vmalloc address value as to what the underlying physical address is. And, FWIW, the page-table walkthrough is far more involved in our xen writeable-page-table kernels than on bare-metal kernels because the physical address encoded into each page-table entry is a *machine* address -- that must be translated back into pseudo-physical address in order to find the next page-table. BTW, for xen unity-mapped address translations, a full page-table walk could be done as well, but it's unnecessary. But I digress... So in any case, the underlying libvirt guest kernel virtual memory access function sounds like it translates them regardless whether they're unity-mapped or vmalloc-mapped addresses, so I think you've answered my question. The issue at hand is that the crash utility is "founded" upon the access of physical-addresses. So all readmem() requests are translated to physical addresses before the resultant value is passed on to /dev/mem or /dev/crash (live systems), or to the myriad of dumpfile formats that crash supports (about a dozen of them). Getting back to the libvirt adaptation into crash, yes, the requests for unity-mapped -- and if I understand it correctly -- for vmalloc addresses, could be passed to your interface directly without a translation to physical. And for crash readmem(PHYSADDR, ...) direct requests, they could presumably be turned into unity-mapped addresses by applying PAGE_OFFSET, and then passing that value to libvirt -- with one significant caveat on 32-bit systems. On 32-bit systems, only the first 896MB of physical memory can be accessed via unity-mapped kernel virtual addresses. The remaining 128MB is given to vmalloc and a handful of other hardwired-type virtual addresses on the top end. So, for example, you wouldn't be able to read user virtual addresses on larger 32-bit systems because their page tables and resultant physical pages are biased to be located in highmem, i.e., above the 896MB limit. That would also be a problem for vmalloc addresses *if* the libvirt interface only handled unity-mapped addresses, which was the genesis of my question. And that's also why I asked about the issue of creating a libvirt interface that accepted pseudo-physical addresses. If that were in place, it would simply mimic /dev/mem (/dev/crash), and it should "just work". There are a couple of other minor gotchas that would also have to be handled for live guest access, like for example, the access of /proc/version. Yep, understood... Right -- and what I'm suggesting is letting the crash utility do all the dirty work for you by just giving crash access to pseudo-physical addresses of the target guest. By doing that, for all practical purposes, crash wouldn't even know that it was even dealing with a "remote" system. Dave