Re: [XEN v1] Xen: Enable compilation when PADDR_BITS == BITS_PER_LONG

[Julien Grall <julien@xxxxxxx>]

On 01/12/2022 12:12, Ayan Kumar Halder wrote:
> On 01/12/2022 10:26, Julien Grall wrote:
> > Hi Ayan,
> Hi Julien,
>
> I have a question.
>
> > On 01/12/2022 10:03, Ayan Kumar Halder wrote:
> > > It is possible for a pointer to represent physical memory of the same 
> > > size.
> > > In other words, a 32 bit pointer can represent 32 bit addressable 
> > > physical
> > > memory.
> > > Thus, issue a compilation failure only when the count of physical 
> > > address bits
> > > is greater than BITS_PER_LONG (ie count of bits in void*).
> > I am having difficult to understand how this description is related to 
> > the BUILD_BUG_ON(). AFAIU, it is used to check that xenheap_bits can 
> > be used in shift.
> > If the unsigned long is 32-bit, then a shift of 32 could be undefined. 
> > Looking at the current use, the shift are used with "xenheap_bits - 
> > PAGE_SHIFT". So as long as PAGE_SHIFT is not 0, you would be fine.
> Ack
> > > Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
> > > ---
> > >
> > > Currently this change will not have any impact on the existing 
> > > architectures.
> > > The following table illustrates PADDR_BITS vs BITS_PER_LONG of 
> > > different archs
> > > ------------------------------------------------
> > > | Arch      |   PADDR_BITS    |   BITS_PER_LONG |
> > > ------------------------------------------------
> > > | Arm_64    |   48            |   64            |
> > > | Arm_32    |   40            |   32            |
> > > | RISCV_64  |   Don't know    |   64            |
> > > | x86       |   52            |   64            |
> > > -------------------------------------------------
> > The Arm_32 line is a bit confusing because one would wonder why we 
> > haven't seen this issue yet. So I think you want to clarify that the 
> > code path is not used by Arm32.
> Do you want this clarification and the above/below explanation to be a 
> part of the commit message ?
No I don't think this will be relevant in the final commit message.

Cheers,

--
Julien Grall