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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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


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