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

From: Boris Ostrovsky
Date: Thu Feb 04 2016 - 14:54:31 EST


On 02/03/2016 06:40 PM, Luis R. Rodriguez wrote:
On Wed, Feb 03, 2016 at 03:11:56PM -0500, Boris Ostrovsky wrote:
On 02/03/2016 01:55 PM, Luis R. Rodriguez wrote:
I saw no considerations for the recommendations I had made last on your v1:

https://lkml.kernel.org/r/CAB=NE6XPA0YzbnM8=rspkKai6d3GkXXO00Gr0VZUYoyzNy6thw@xxxxxxxxxxxxxx

Of importance:

1) Using pv_info.paravirt_enabled = 1 is wrong unless you mean to say this
is for legacy x86:

Your patch #3 keeps on setting pv_info.paravirt_enabled = 1 and as discussed
this is wrong. It will be renamed to x86_legacy_free() to align with what folks
are pushing for a BIOS flag to annotate if a system requires legacy x86 stuff.
This also means re-thinking all use cases and ensuring subarch is used then
instead when the goal was to avoid Xen from entering that code. Today Xen does
not use this but with my work it does and it helps clean and brush up a lot of
these checks with future prospects to even help unify entry points.
As I said earlier, I am not sure I understand what subarch buys us
for HVMlite guests.
I accepted subarch may not be the right thing, so proposed a hypervisor type.

I don't see much difference between having an HV-specific subarch and a hypervisor type.

What it buys you is a strong semantics association between code designed
for a purpose.

As for using paravirt_enabled -- this is really only used to
differentiate HVM from HVMlite and I think (although I'd need to
check) is only needed by Xen-specific code in a couple of places.
That sounds like a Xen specific use case as such an interface that is
pointed out as going to renamed to reflect its actual use case should not
be abused for that purpose.

So if/when it is removed we will switch to something else. Since your work is
WIP I decided to keep using it until it's clear what other options may be
available.
And your work is not WIP? I'll be splitting my patches up and the rename
will be atomic, it likely can go in first than yours, so not sure why you
are simply brushing this off.

I didn't mean to imply anything by saying that your patches are a WIP. It's just that I can only write and test my patches against existing code, not the future one.

I am sorry if you felt I was trying to say something else, it certainly was not my intent.


2) We should avoid more hypervisor type hacks, and just consider a new
hypervisor type to close the gap:

Using x86_legacy_free() and friends in a unified way for all systems means it
should only be used after init_hypervisor_platform() which is called during
setup_arch(). This means we have a semantic gap for checks on "are we on
hypervisor type and which one?".
In this particular case we don't need any information about
hypervisor until init_hypervisor_platform().
I pointed out in your v1 patchset how microcode loading was not blocked, you
then asked how KVM does it, and that was explained as well, and that they
don't enable it as well. You need a solution for this.

Not really. Xen will ignore writes to microcode-specific MSRs, just like KVM.

This is exact same behavior we have now with regular HVM guests.


As-is the x86 boot protocol would not allow an easy way for this, I'm suggesting we consider extending the boot protocol to add a hypervisor type and data pointer much as with subarch and subarch_data for the

Who will set hypervisor type and where? It won't be Xen as Andrew mentioned in another email.

particular purpose of both enabling entry into the same startup_32()
but also a clean way for modifications of stubs both at the beginning
and at the end of startup_32().

Pseudo code:

startup_32() startup_64()
| |
| |
V V
pre_hypervisor_stub_32() pre_hypervisor_stub_64()
| |
| |
V V
[existing startup_32()] [existing startup_64()]
| |
| |
V V
post_hypervisor_stub_32() post_hypervisor_stub_64()

The pre_hypervisor_stub_32() would have much of the code in
hvmlite_start_xen() but for 32-bit, pre_hypervisor_stub_64()
would have the 64-bits.


Sure. When the protocol is agreed upon and this code is written we will just move hvmlite_start_xen() to pre_hypervisor_stub_32().



+int xen_hvmlite __attribute__((section(".data"))) = 0;
+struct hvm_start_info hvmlite_start_info __attribute__((section(".data")));
+uint hvmlite_start_info_sz = sizeof(hvmlite_start_info);
+struct boot_params xen_hvmlite_boot_params __attribute__((section(".data")));
+#endif
+
The section annotations seems very special use case but likely worth documenting
and defining a new macro for in include/linux/compiler.h. This would make it
easier to change should we want to change the section used here later and
enable others to easily look for the reason for these annotations in a
single place.
I wonder whether __initdata would be a good attribute. We only need
this early in the boot.
I could not find other users of .data other than some specific driver.
Using anything with *init* alludes you can free the data later but if we
want to keep it I suggest a different prefix, up to you.

That's why I said that we only need this info early in the boot.

-boris