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Re: [Minios-devel] [UNIKRAFT PATCHv4 22/43] plat/kvm: Allow access to floating-point and Advanced SIMD registers


On 07/09/2018 12:48 PM, Julien Grall wrote:
(+ Dave)

On 09/07/18 10:47, Wei Chen wrote:
Hi, Julien,

Hi Wei,

-----Original Message-----
From: Julien Grall <julien.grall@xxxxxxx>
Sent: 2018年7月8日 6:17
To: Wei Chen <Wei.Chen@xxxxxxx>; minios-devel@xxxxxxxxxxxxxxxxxxxx;
Cc: Kaly Xin <Kaly.Xin@xxxxxxx>; nd <nd@xxxxxxx>
Subject: Re: [Minios-devel] [UNIKRAFT PATCHv4 22/43] plat/kvm: Allow access to
floating-point and Advanced SIMD registers


On 07/06/2018 10:03 AM, Wei Chen wrote:
On Arm64, when we want to print out some message, the va_list will
use the SIMD&FP registers (like q0, q1) to store parameters. So, we
have to disable the trap of accessing floating-point and Advanced SIMD
registers to make va_list use SIMD&FP registers properly.

I don't think this is true. Xen and Linux don't use SIMD&FP registers as
this would require to context switch at every entry. Yet we are able to
use va_list without SIMD&FP registers.

This is true, because Linux and Xen doesn't support float print.
Unikraft kernel just a part of an application, its print library need
To support float. Just like nolibc, when GCC compiling this library,
it doesn't care you are running EL1 or EL0, the generated code will
access q0, q1. When you're running this code in EL1 without SIMD&FP,
system crash.

According to the [2] and [4], Cortex-A53 could be configured and linux[3] has support for running the soft floating point. But in our description of Cortex-A53 in "arch/arm/arm64/Config.uk" we restrict it.

Do we plan to support it in the future?

When will q0, q1 will be accessed? Is it only when floating point will
be printed?

The document[1], mentions that register qN are used only for floating and vector operations. In appendix B, where they explain about the variadic calling conventions it mentions prologue of the variadic function creates a separate variadic stack frame and if the Floating points arguments or vector arguments are not passed as a variadic argument qN register do not have to be saved and restored.

For further reference, section 5.4[1] describe how the function parameters are passed and how the SIMD/floating point register are used. Appendix B of the document gives a description of variadic functions.

Also, IIUC, the kernel code will be compiled with floating point, right?
If so, it means the compiler will be able to use floating point for some
optimization. This will become a problem when receiving a trap as you
would need to context switch the registers (this assume handler may use FP).


[1] http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055b/IHI0055B_aapcs64.pdf

[2] https://developer.arm.com/products/architecture/cpu-architecture/a-profile/docs/112086438/latest/software-implications-for-v8-a-implementations-with-no-hardware-floating-point

[3] https://patchwork.kernel.org/patch/9405787/

[4] http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0773b/chr1383143713787.html

Thanks & Regards

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