Re: [Minios-devel] [UNIKRAFT RFC PATCH 2/5] plat/kvm: arm64: Enable the fp/simd at the starting point

[Julien Grall <julien@xxxxxxx>]

On 20/12/2019 13:10, Santiago Pagani wrote:
> Hi Julien,
Hi,

Thank you for not top-posting, but please configure your e-mail client 
to avoid quoting with tab. After a few e-mails the thread will be a 
nightmare to read.
> On 20.12.19, 08:50, "Julien Grall" <julien@xxxxxxx> wrote:
> I've been thinking a bit more about this, and I do not think we can do what you 
> suggest. Namely, in Unikraft there is no difference between a kernel and an 
> application. This is not Linux, where we have user space and applications and 
> kernel space.
The concept of Unikraft is not much different from drivers/code in Linux 
kernel (such as crypto) able to use FPSIMD but not the rest of the 
kernel (see kernel_neon_{begin, end}).
While your application and kernel is linked in the same binary,  you 
still have some boundary between kernel and application code. So you 
could decide when to enable/disable FPSIMD.
> Everything gets compiled as a single image, running in flat memory space, everything with the same privileges and 
> properties. We could compile the "application" files without the -mgeneral-regs-only flag, and the 
> "kernel" files with it, but that does not guarantee that there will not be corruption in an exception, as the 
> exception could occur when running "application code", and it is directly handled without going through the 
> "kernel".
Well if your exception is free of FPSIMD instructions then you don't 
have any corruption problem.
> Therefore, when we are compiling an image that has floating point support, then 
> yes, every will need to save/restore all registers that could potentially get 
> corrupted, and this should be added by Justin to his patch.
>

So it means that every time you receive an interrupt, you will have to 
save/restore the FPSIMD. That's going to increase to increase quite a 
bit the cost of receiving an interrupt.
If that's acceptable for you, then fine.

> However, we need consistency for a given imagen, and we cannot have different 
> parts of the image compiled with/without the -mgeneral-regs-only flag.
Why so? It is just a matter how you define your boundary.

Cheers,

--
Julien Grall

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