Re: [PATCH v3 18/28] arm64/sve: Preserve SVE registers around EFI runtime service calls

Catalin Marinas
2017-10-12T10:56:27Z

On Tue, Oct 10, 2017 at 07:38:35PM +0100, Dave P Martin wrote:
> The EFI runtime services ABI allows EFI to make free use of the
> FPSIMD registers during EFI runtime service calls, subject to the
> callee-save requirements of the AArch64 procedure call standard.
>
> However, the SVE architecture allows upper bits of the SVE vector
> registers to be zeroed as a side-effect of FPSIMD V-register
> writes. This means that the SVE vector registers must be saved in
> their entirety in order to avoid data loss: non-SVE-aware EFI
> implementations cannot restore them correctly.
>
> The non-IRQ case is already handled gracefully by
> kernel_neon_begin(). For the IRQ case, this patch allocates a
> suitable per-CPU stash buffer for the full SVE register state and
> uses it to preserve the affected registers around EFI calls. It is
> currently unclear how the EFI runtime services ABI will be
> clarified with respect to SVE, so it safest to assume that the
> predicate registers and FFR must be saved and restored too.
>
> No attempt is made to restore the restore the vector length after
> a call, for now. It is deemed rather insane for EFI to change it,
> and contemporary EFI implementations certainly won't.
>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>