Re: [Qemu-devel] QEMU/NEMU boot time with several x86 firmwares

[Boris Ostrovsky]

On 12/5/18 8:20 AM, Stefan Hajnoczi wrote:
> On Tue, Dec 04, 2018 at 02:44:33PM -0800, Maran Wilson wrote:
>>
>> Since then, we have put together an alternative solution that would allow
>> Qemu to boot an uncompressed Linux binary via the x86/HVM direct boot ABI
>> (https://xenbits.xen.org/docs/unstable/misc/pvh.html). The solution involves
>> first making changes to both the ABI as well as Linux, and then updating
>> Qemu to take advantage of the updated ABI which is already supported by both
>> Linux and Free BSD for booting VMs. As such, Qemu can remain OS agnostic,
>> and just be programmed to the published ABI.
>>
>> The canonical definition for the HVM direct boot ABI is in the Xen tree and
>> we needed to make some minor changes to the ABI definition to allow KVM
>> guests to also use the same structure and entry point. Those changes were
>> accepted to the Xen tree already:
>> https://lists.xenproject.org/archives/html/xen-devel/2018-04/msg00057.html
>>
>> The corresponding Linux changes that would allow KVM guests to be booted via
>> this PVH entry point have already been posted and reviewed:
>> https://lkml.org/lkml/2018/4/16/1002
>>
>> The final part is the set of Qemu changes to take advantage of the above and
>> boot a KVM guest via an uncompressed kernel binary using the entry point
>> defined by the ABI. Liam Merwick will be posting some RFC patches very soon
>> to allow this.
> Cool, thanks for doing this work!
>
> How do the boot times compare to qemu-lite and Firecracker's
> (https://github.com/firecracker-microvm/firecracker/) direct vmlinux ELF
> boot?
>
> I'm asking because there are several custom approaches to fast kernel
> boot and we should make sure that whatever Linux and QEMU end up
> natively supporting is likely to work for all projects (NEMU, qemu-lite,
> Firecracker) and operating systems (Linux distros, other OSes).


I should also add that effort is under way to add support for booting
PVH guests from grub. This is currently Xen guests only (obviously) but
since it's based on the ABI that Maran mentioned above I don't see why a
non-Xen guest can't be supported as well.

v6 is here:
https://lists.xenproject.org/archives/html/xen-devel/2018-11/msg03174.html


-boris

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