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Re: [PATCH v7 09/12] xen: add runtime parameter access support to hypfs

On 14/04/2020 10:31, Jan Beulich wrote:
On 14.04.2020 11:29, Julien Grall wrote:
On 03/04/2020 16:31, Jürgen Groß wrote:
On 03.04.20 16:51, Jan Beulich wrote:
On 02.04.2020 17:46, Juergen Gross wrote:
- fine tune some parameter initializations (Jan Beulich)
- call custom_runtime_set_var() after updating the value
- modify alignment in Arm linker script to 4 (Jan Beulich)

I didn't ask for this to be unilaterally 4 - I don't think this
would work on Arm64, seeing that there are pointers inside the
struct. This wants to be pointer size, i.e. 4 for Arm32 but 8
for Arm64.

We don't allow unaligned access on Arm32, so if your structure happen to have a 
64-bit value in it then you will get a crash at runtime.

For safety, it should neither be POINTER_ALIGN or 4, but 8.
This is going to make your linker more robust.

Would you mind explaining to me why POINTER_ALIGN would be wrong
when the most strictly aligned field in a structure is a pointer?
Both are valid with one difference though. If tomorrow someone send a patch to add a 64-bit in the structure, what are the chance one won't notice the alignment change? It is quite high.

If you align the section to 8, then you make your code more robust at the expense of possibly adding an extra 4-bytes in your binary.


Julien Grall



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