VMCoreInfo device

The -device vmcoreinfo will create a fw_cfg entry for a guest to store dump details.


A guest may use this fw_cfg entry to add information details to QEMU dumps.

The entry of 16 bytes has the following layout, in little-endian:


struct FWCfgVMCoreInfo {
    uint16_t host_format;  /* formats host supports */
    uint16_t guest_format; /* format guest supplies */
    uint32_t size;         /* size of vmcoreinfo region */
    uint64_t paddr;        /* physical address of vmcoreinfo region */

Only full write (of 16 bytes) are considered valid for further processing of entry values.

A write of 0 in guest_format will disable further processing of vmcoreinfo entry values & content.

You may write a guest_format that is not supported by the host, in which case the entry data can be ignored by QEMU (but you may still access it through a debugger, via vmcoreinfo_realize::vmcoreinfo_state).

Format & content

As of QEMU 2.11, only VMCOREINFO_FORMAT_ELF is supported.

The entry gives location and size of an ELF note that is appended in qemu dumps.

The note format/class must be of the target bitness and the size must be less than 1Mb.

If the ELF note name is VMCOREINFO, it is expected to be the Linux vmcoreinfo note (see the kernel documentation for its format). In this case, qemu dump code will read the content as a key=value text file, looking for NUMBER(phys_base) key value. The value is expected to be more accurate than architecture guess of the value. This is useful for KASLR-enabled guest with ancient tools not handling the VMCOREINFO note.