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

[Ilya Leoshkevich]

On 9/28/26 18:15, Steffen Eiden wrote:
> On Mon, Sep 28, 2026 at 05:53:33PM +0200, Ilya Leoshkevich wrote:
> > 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@xxxxxxxxxxxxx>
> > > Signed-off-by: Andreas Grapentin <gra@xxxxxxxxxxxxx>
> > > Co-developed-by: Nina Schoetterl-Glausch <nsg@xxxxxxxxxxxxx>
> > > Signed-off-by: Nina Schoetterl-Glausch <nsg@xxxxxxxxxxxxx>
> > > Signed-off-by: Steffen Eiden <seiden@xxxxxxxxxxxxx>
> > > ---
> > >    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

[...]
> > > +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?
> yes. Also, some later (unsent) patch needs this and before I forget this, let's
> do it now.

Thanks for the clarification!

Reviewed-by: Ilya Leoshkevich <iii@xxxxxxxxxxxxx>

with typos fixes.