今天看到公司內部的backport過了一遍，讓KVM支持啓動PVH的guest，挺有意思的功能，把啓動時間的優化做到了極致，不走bios，qboot直接建立memory map，並跳轉到kernel入口地址。這個入口不同於native入口地址，在upstream裏是pvh_start_xen， 通過下面的elfnote導出。
ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY, _ASM_PTR (pvh_start_xen - __START_KERNEL_map))
ABI的細節可以看 https://xenbits.xen.org/docs/unstable/misc/pvh.html
同事做的性能比較，運行到內核入口的時間少了一半！
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)