Re: [Xen-devel] [PATCH v2 02/11] xen/hvmlite: Bootstrap HVMlite guest

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/02/2016 23:59, Luis R. Rodriguez wrote:
> On Wed, Feb 03, 2016 at 08:52:50PM +0000, Andrew Cooper wrote:
>> On 03/02/16 18:55, Luis R. Rodriguez wrote:
>>> We add new hypervisor type to close the semantic gap for hypervisor types, 
>>> and
>>> much like subarch enable also a subarch_data to let you pass and use your
>>> hvmlite_start_info. This would not only help with the semantics but also 
>>> help
>>> avoid yet-another-entry point and force us to provide a well define 
>>> structure
>>> for considering code that should not run by pegging it as required or 
>>> supported
>>> for different early x86 code stubs.
>> Was I unclear last time?  Xen *will not* be introducing Linux-specifics
>> into the HVMLite starting ABI.
> This does not have to be "Linux specifics" but rather a light way to enable
> a hypervisor to clue in *any* OS of its hypervisor type, guest type, and
> custom hypervisor data that can be used to populate needed OS specifics
> about the guest. Perhaps Xen's own loader mechanism could be extended just
> slightly to become *that* standard, its just right now it doesn't seem to
> enable for generalizing this in a very useful way for OSes. Its all
> custom stubs.

There are already standard x86 ways of doing this, via the hypervisor
cpuid bits.  Xen presents itself normally in this regard, as do all the
other hypervisors.

It is completely backwards to expect a hypervisor (or toolstack in our
case) to deliberately prod what it suspects might be a Linux binary in a
way which it things a Linux binary might like to be prodded.

>
>> Your perceived problem with multiple entry points is not a problem with
>> multiple entry points; It is a problem with multiple different paths
>> performing the same initialisation.
> Its actually more of an issue with the lack of strong general semantics
> available for different hypervisors and guest types and requirements for x86's
> init path. What you end up with as collateral is multiple entry points, and
> these can be sloppy and as you note can perform the same initialisation.
> Another issue is the inability to proactively ensure new x86 init code
> addresses different x86 requirements (cr4 shadow regression and Kasan still
> being broken on Xen are two examples) and it just so happens that the lack of
> semantics for the different guest types required to be evaluated is one issue
> for x86.
>
> We can do better.

Even with a perfect startup() routine which caters for all runtime
usecases, you cannot avoid having multiple entry stubs to cater for the
different ways the binary might be started.

Unless you are volunteering to write a single stub which can first
evaluate whether it is in 16/32/64bit mode, then create a safe stack to
use, then evaluate how it was started (multiboot, legacy BIOS, EFI,
etc.) and turn all this information into a zeropage.

I don't know that would be possible, but the point is moot as it
definitely wouldn't be maintainable if it were possible.

>
>> The Linux entry for PV guests is indeed completely horrible.  I am not
>> trying to defend it in the slightest.
>>
>> However, the HVMLite entry which is a very short stub that sets up a
>> zeropage and hands off to the native start routine is fine.
> Its alright, and a huge stepping stone towards good architecture. We
> however can do better.

Then we are generally in agreement.  However, at the risk of sounding
like a grump old sod,  take this win first and then work on the next
stepping stone.

Review comments identifying "I am working on implementing a new $X which
will make this area better/easier/more shiny in the future" are fine. 
Review comments complaining that "you haven't used this shiny new $X
which doesn't exist yet" are a waste of time; time which you would be
better spent implementing said $X.

Noone is disagreeing that improvements can be made, but don't try to do
them all at once, or nothing will get done.

>
>> There is still just routine performing native x86 startup.
>>
>> If you still desperately want to avoid multiple entry points, then just
>> insist on using grub for the VM.  I expect that that is how most people
>> will end up using HVMLite VMs anyway.
> Are you saying Grub can do some of this heavy lifting that I am trying
> to avoid? If so that'd be great news.

There are two different ways of running your VM, depending on your usecase.

The more traditional approach of a full OS will want to load a kernel
out of the guests filesystem, according to the guests bootloader
configuration.  At this point it doesn't matter for Linux as it will be
booted by some already-existing bootloader which already knows how to do
the job.

The more container/unikernel oriented approach is to boot an image
directly from dom0, skipping the middle layers of firmware and
filesystems.  For this to work, Linux needs to be able to be started via
the hypervisor ABI of choice, which in this case is something which
looks very much like multiboot.

~Andrew

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