QEMU + qboot + PVH vmlinux

今天看到公司内部的backport过了一遍，让KVM支持启动PVH的guest，挺有意思的功能，把启动时间的优化做到了极致，不走bios，qboot直接建立memory map，并跳转到kernel入口地址。这个入口不同于native入口地址，在upstream里是pvh_start_xen， 通过下面的elfnote导出。

ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY, _ASM_PTR (pvh_start_xen - __START_KERNEL_map))

ABI的细节可以看 https://xenbits.xen.org/docs/unstable/misc/pvh.html

同事做的性能比较，运行到内核入口的时间少了一半！

QEMU + qboot + vmlinux (PVH + 4.20-rc4)
 qemu_init_end: 41.550521
 fw_start: 41.667139 (+0.116618)
 fw_do_boot: 47.448495 (+5.781356)
 linux_startup_64: 47.720785 (+0.27229)
 linux_start_kernel: 48.399541 (+0.678756)
 linux_start_user: 296.952056 (+248.552515)

QEMU + qboot + bzImage:
 qemu_init_end: 29.209276
 fw_start: 29.317342 (+0.108066)
 linux_start_boot: 36.679362 (+7.36202)
 linux_startup_64: 94.531349 (+57.851987)
 linux_start_kernel: 94.900913 (+0.369564)
 linux_start_user: 401.060971 (+306.160058)

QEMU + bzImage:
 qemu_init_end: 30.424430
 linux_startup_64: 893.770334 (+863.345904)
 linux_start_kernel: 894.17049 (+0.400156)
 linux_start_user: 1208.679768 (+314.509278)