Re: [Xen-devel] [PATCH 9/9] x86: move declaration of arch_set_info_hvm_guest and provide stub

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 16.08.18 at 14:59, <wei.liu2@xxxxxxxxxx> wrote:
> On Thu, Aug 16, 2018 at 05:24:15AM -0600, Jan Beulich wrote:
>> >>> On 16.08.18 at 12:42, <wei.liu2@xxxxxxxxxx> wrote:
>> > On Tue, Aug 07, 2018 at 05:24:15PM +0100, Wei Liu wrote:
>> >  > 
>> >> > All uses sit either in HVM-specific code or inside is_hvm_...()
>> >> > conditionals: Why do we need the inline stub? If the declaration
>> >> > was visible independent of CONFIG_HVM, code would compile
>> >> > fine, and references to the function would be removed by the
>> >> > compiler, so linking would also succeed despite there not being
>> >> > any definition of the function.
>> >> 
>> >> Last time I tried DCE wasn't working so well. Let me try again and if
>> >> DCE works it would save me a lot of effort to provide stubs.
>> >> 
>> > 
>> > DCE seems to work better this time.
>> > 
>> > The only problem is that some ASSERTs will need to turn into panic or
>> > BUG() if we want to fully utilise DCE. Is that okay?
>> 
>> In general yes, I think so.
>> 
>> > To give you an example, after making is_hvm_domain evaluate to 0 when
>> > !CONFIG_HVM, vm_event_fill_regs + !CONFIG_HVM compiles fine for debug
>> > build because ASSERT hints the compiler that the rest of the function is
>> > never going to be reachable. But for non-debug build, ASSERT is empty,
>> > so compiler will not eliminate the rest of the function, complaining
>> > hvm_get_segment_register is not available. It is solvable by adding
>> > panic or BUG.
>> > 
>> > There is going to be quite a few cases like that. I haven't gone through
>> > all of them.
>> 
>> In cases like the example you give I'm not convinced of the
>> suggested conversion though - the entire function should then
>> live inside CONFIG_HVM (or in a file built for HVM only).
>> 
> 
> This will do, too -- if you don't mind littering CONFIG_HVM in files.
> 
> VM event subsystem is entangled with other subsystems, too, so it will
> take some time to clean that up. I haven't got to that part yet. At the
> moment I have accumulated ~25 patches to almost make !CONFIG_HVM work
> for debug build. I will go through all patches later to make them work
> with non-debug build.

That'll be fine for now, I think. Eventually the HVM pieces should be
moved to hvm/ of course.

> One thing I haven't decided what to do is hvm/i8254.c, which is used by
> both PV and HVM. I'm thinking about moving that file under arch/x86 and
> rename it emul-i8254.c. Is that a sensible thing to do?

Any chance you could leave HVM-only parts where they are? Or
would that make more of a mess than moving the entire file?

Jan



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