Re: [Xen-devel] [PATCH 01/14] xen: arm: define 64-bit guest hypercall calling convention.

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 12 Mar 2013, Ian Campbell wrote:
> From: Ian Campbell <ian.campbell@xxxxxxxxxx>
> 
> As well as using x<N> rather than r<N> registers for passing arguments/results
> as mandate the use of x16 as the hypercall number.
> 
> Add some pedantry about struct alignment layout referencing the ARM Procedure
> Calling Standard to avoid confusion with the previous "OABI" convention. While
> at it also mandate that hypercall argument structs are always little endian.
> 
> Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

Acked-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>


>  xen/include/public/arch-arm.h |   27 +++++++++++++++++++--------
>  1 file changed, 19 insertions(+), 8 deletions(-)
> 
> diff --git a/xen/include/public/arch-arm.h b/xen/include/public/arch-arm.h
> index 3333399..e44e90f 100644
> --- a/xen/include/public/arch-arm.h
> +++ b/xen/include/public/arch-arm.h
> @@ -33,20 +33,31 @@
>   * A hypercall is issued using the ARM HVC instruction.
>   *
>   * A hypercall can take up to 5 arguments. These are passed in
> - * registers, the first argument in r0, the second argument in r1, the
> - * third in r2, the forth in r3 and the fifth in r4.
> + * registers, the first argument in x0/r0 (for arm64/arm32 guests
> + * respectively irrespective of whether the underlying hypervisor is
> + * 32- or 64-bit), the second argument in x1/r1, the third in x2/r2,
> + * the forth in x3/r3 and the fifth in x4/r4.
>   *
> - * The hypercall number is passed in r12.
> + * The hypercall number is passed in r12 (arm) or x16 (arm64). In both
> + * cases the relevant ARM procedure calling convention specifies this
> + * is an inter-procedure-call scratch register (e.g. for use in linker
> + * stubs). This use does not conflict with use during a hypercall.
>   *
>   * The HVC ISS must contain a Xen specific TAG: XEN_HYPERCALL_TAG.
>   *
> - * The return value is in r0.
> + * The return value is in x0/r0.
>   *
> - * The hypercall will clobber r12 and the argument registers used by
> - * that hypercall (except r0 which is the return value) i.e. a 2
> - * argument hypercall will clobber r1 and a 4 argument hypercall will
> - * clobber r1, r2 and r3.
> + * The hypercall will clobber x16/r12 and the argument registers used
> + * by that hypercall (except r0 which is the return value) i.e. in
> + * addition to x16/r12 a 2 argument hypercall will clobber x1/r1 and a
> + * 4 argument hypercall will clobber x1/r1, x2/r2 and x3/r3.
>   *
> + * Parameter structs passed to hypercalls are laid out according to
> + * the Procedure Call Standard for the ARM Architecture (AAPCS, AKA
> + * EABI) and Procedure Call Standard for the ARM 64-bit Architecture
> + * (AAPCS64). Where there is a conflict the 64-bit standard should be
> + * used regardless of guest type. Structures which are passed as
> + * hypercall arguments are always little endian.
>   */
>  
>  #define XEN_HYPERCALL_TAG   0XEA1
> -- 
> 1.7.10.4
> 

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