[PATCH v7 1/2] arm64/sve: Split TIF_SVE into separate execute and register state flags

[Dave Martin]

On Mon, Feb 01, 2021 at 12:29:00PM +0000, Mark Brown wrote:
> Currently we have a single flag TIF_SVE which says that a task is
> allowed to execute SVE instructions without trapping and also that full
> SVE register state is stored for the task.  This results in us doing
> extra work storing and restoring the full SVE register state even in
> those cases where the ABI is that only the first 128 bits of the Z0-V31
> registers which are shared with the FPSIMD V0-V31 are valid.
> 
> In order to allow us to avoid these overheads split TIF_SVE up so that
> we have two separate flags, TIF_SVE_EXEC which allows execution of SVE
> instructions without trapping and TIF_SVE_FULL_REGS which indicates that
> the full SVE register state is stored.  If both are set the behaviour is
> as currently, if TIF_SVE_EXEC is set without TIF_SVE_FULL_REGS then we
> save and restore only the FPSIMD registers until we return to userspace
> with TIF_SVE_EXEC enabled at which point we convert the FPSIMD registers
> to SVE.  It is not meaningful to have TIF_SVE_FULL_REGS set without
> TIF_SVE_EXEC.
> 
> This patch is intended only to split the flags, it does not take
> avantage of the ability to set the flags independently and the new
> state with TIF_SVE_EXEC only should not be observed.
> 
> This is based on earlier work by Julien Gral implementing a slightly
> different approach.
> 
> Signed-off-by: Mark Brown <broonie at kernel.org>

While I'm trying to get my head around this, can you sketch out the
mapping between the old flag combinations (including Julien's
TIF_SVE_NEEDS_FLUSH) and yours?

[...]

Cheers
---Dave