Re: [Xen-devel] [PATCH v6 08/15] x86/efi: create new early memory allocator

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.09.16 at 12:52, <daniel.kiper@xxxxxxxxxx> wrote:
> On Tue, Sep 20, 2016 at 03:57:19AM -0600, Jan Beulich wrote:
>> >>> On 20.09.16 at 11:45, <daniel.kiper@xxxxxxxxxx> wrote:
>> > On Mon, Sep 19, 2016 at 09:17:50AM -0600, Jan Beulich wrote:
>> >> >>> On 19.09.16 at 17:04, <daniel.kiper@xxxxxxxxxx> wrote:
>> >> > On Mon, Sep 19, 2016 at 06:12:35AM -0600, Jan Beulich wrote:
>> >> >> >>> On 12.09.16 at 22:18, <daniel.kiper@xxxxxxxxxx> wrote:
>> >> >> > --- a/xen/arch/x86/setup.c
>> >> >> > +++ b/xen/arch/x86/setup.c
>> >> >> > @@ -520,6 +520,8 @@ static void noinline init_done(void)
>> >> >> >
>> >> >> >      system_state = SYS_STATE_active;
>> >> >> >
>> >> >> > +    free_ebmalloc_unused_mem();
>> >> >>
>> >> >> Now that the allocator properly lives in common code, this appears
>> >> >> to lack an ARM side counterpart.
>> >> >
>> >> > Why? It is called only from xen/arch/x86/setup.c:__start_xen() and all
>> >> > ebmalloc stuff is in #ifndef CONFIG_ARM. So, free_ebmalloc_unused_mem()
>> >> > will be needed only if we add ARM support here.
>> >>
>> >> Well, it being inside that conditional is part of the problem - there's
>> >> no apparent point for all of it to be.
>> >
>> > I can agree that this is potentially generic stuff (well, I understand that
>> > it is our final goal but unreachable yet due to various things). However,
>> > right know it is only used on x86. So, I am not sure what is the problem
>> > with #ifndef CONFIG_ARM right now...
>>
>> It is a fact that these should actually not be there, so we ought to
>> at least limit them to the smallest possible count and scopes.
>>
>> >> Arguably the one static function may better be, as other workarounds
>> >> to avoid the "unused" compiler warning wouldn't be any better.
>> >
>> > Do you mean static function with empty body for ARM? It is not needed
>> > right now because it is never called on ARM. Am I missing something?
>>
>> You misunderstood - I said that for this one (unused) static
>> function such an #ifdef is probably okay, as the presumably
>> smallest possible workaround.
> 
> Do you suggest that I should move out of #ifndef CONFIG_ARM all ebmalloc stuff
> except free_ebmalloc_unused_mem(). Even if it is not used on ARM right now?

That's not the static function, is it? The function you name should
actually be called on ARM too (as I did point out elsewhere already),
just to not leave a trap open for someone to fall into when (s)he
adds a first use of the allocator on ARM.

>> >> >> > +static unsigned long __initdata ebmalloc_allocated;
>> >> >> > +
>> >> >> > +/* EFI boot allocator. */
>> >> >> > +static void __init *ebmalloc(size_t size)
>> >> >> > +{
>> >> >> > +    void *ptr = ebmalloc_mem + ebmalloc_allocated;
>> >> >> > +
>> >> >> > +    ebmalloc_allocated += (size + sizeof(void *) - 1) & 
>> >> >> > ~((typeof(size))sizeof(void *) - 1);
>> >> >>
>> >> >> What's the point of this ugly cast?
>> >> >
>> >> > In general ALIGN_UP() would be nice here. However, there is no such 
>> >> > thing
>> >> > in Xen headers (or I cannot find it). Should I add one? As separate 
>> >> > patch?
>> >>
>> >> I understand what you want the expression for, but you didn't
>> >> answer my question. Or do you not realize that all this cast is
>> >> about is a strange way of converting an expression of type
>> >> size_t to type size_t?
>> >
>> > Does sizeof() returns size_t type? I was thinking that it returns
>> > a number calculated during compilation, however, it does not have
>> > specific type.
>>
>> Every expression needs to have a well specified type. Even
>> plain numbers do.
> 
> Hmmm... So, what is a type e.g. 5 without "U" and/or "L"? int?

Of course. But please may I ask you to read the spec instead?

Jan


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