[RFC patch 3/5] RISC-V: Save and restore VCSR when doing user context switch

[Andrew Waterman]

On Tue, Sep 14, 2021 at 4:48 PM Joseph Myers <joseph@codesourcery.com> wrote:
>
> On Mon, 13 Sep 2021, Vincent Chen wrote:
>
> > According to the RISC-V V extension specification, all vector registers
> > except VCSR are caller-saved registers. The VCSR (vxrm + vxsat) has thread
> > storage duration. Therefore, only VCSR needs to be added to the user
> > context operation.
>
> What is the intended programming model for using vxrm and vxsat?
>
> The expectation for the floating-point rounding modes and flags is that
> they work just like user-defined _Thread_local variables - that is, they
> are *not* saved or restored by setjmp/longjmp or *context functions.  It
> would be natural to expect fixed-point rounding modes and flags to work
> similarly.

Indeed, Joseph, vxsat and vxrm should be treated analogously to the FP
flags and rounding mode, respectively.

>
> --
> Joseph S. Myers
> joseph@codesourcery.com