Re: [PATCH v8 20/29] s390: Introduce Start Arm Execution instruction

[Ilya Leoshkevich <iii@linux.ibm.com>]

On 9/18/26 15:30, Steffen Eiden wrote:
> The Start Arm Execution (SAE) instruction is the centerpiece for
> executing arm64 (KVM) guests on s390. Its purpose is, similar to SIE, to
> enable accelerated execution of arm64 virtual machines. SAE expects the
> physical address of a control block as the only argument.
> 
> The host is responsible to save & restore
>   - GPRs 0-13
>   - access register 0-15
              ^^^^^^^^
              registers>   - breaking event register (BEAR)
>   - vector/floating point registers
> between SAE executions to guarantee host consistency.
> 
> GPRs and BEAR are save and restores in the asm functions. The other
                     ^^^^^^^^^^^^^^^^^
                     saved and restored

> register are handled in within C code. Access registers are handled in a
   ^^^^^^^^             ^^
   registers            not needed probably?

> later patch and SVEs will be handled when they are introduced in a
> future series. Most arm64 registers are handled by a satellite block
> called save_area. Some registers, frequently used by hypervisors, are
> placed into the SAE control block itself.
> 
> Enlighten asm/kvm_host_types.h for the new header variant. The new
> header is chosen instead of asm/kvm_host_s390_types.h if KVM_S390_ARM64
> is defined.
> 
> Co-developed-by: Andreas Grapentin <gra@linux.ibm.com>
> Signed-off-by: Andreas Grapentin <gra@linux.ibm.com>
> Co-developed-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
> Signed-off-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
> Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
> ---
>   arch/s390/include/asm/asm-prototypes.h       |   1 +
>   arch/s390/include/asm/kvm_host_arm64_types.h | 124 +++++++++++++++++++
>   arch/s390/include/asm/kvm_host_types.h       |   4 +
>   arch/s390/include/asm/sae.h                  |  58 +++++++++
>   arch/s390/include/asm/stacktrace.h           |   5 +
>   arch/s390/kernel/asm-offsets.c               |   1 +
>   arch/s390/kernel/entry.S                     |  22 ++++
>   arch/s390/tools/opcodes.txt                  |   3 +
>   8 files changed, 218 insertions(+)
>   create mode 100644 arch/s390/include/asm/kvm_host_arm64_types.h
>   create mode 100644 arch/s390/include/asm/sae.h

[...]
    > +#if IS_ENABLED(CONFIG_KVM)
> +/*
> + * __sae64a calling convention:
> + * %r2 pointer to sae control block physical address
           ^^^^^^^^^^ I guess it's just a physical address and not a
                      pointer to it?

                      ^^^ sometimes it's called "sae", sometimes "SAE" -
                          it would be nice to have some consistency> + */
> +SYM_FUNC_START(__sae64a)
> +	stmg	%r6,%r14,__SF_GPRS(%r15)	# store kernel registers

Strictly speaking we don't need to save %r14, but I guess you do this
for unwinding?

> +	STBEAR	__SF_SAE_BEAR(%r15)		# save breaking event address register
> +	.insn	rre,0xb9a50000,%r2,0		# Start Arm Execution
> +	LBEAR	__SF_SAE_BEAR(%r15)		# restore breaking event address register
> +	lmg	%r6,%r14,__SF_GPRS(%r15)	# restore kernel registers
> +	xgr	%r0,%r0				# clear guest registers to
> +	xgr	%r1,%r1				# prevent speculative use
> +	xgr	%r2,%r2
> +	xgr	%r3,%r3
> +	xgr	%r4,%r4
> +	xgr	%r5,%r5
> +	BR_EX	%r14
> +SYM_FUNC_END(__sae64a)
> +EXPORT_SYMBOL(__sae64a)
> +#endif
[...]