Re: [PATCH v7 13/27] KVM: arm64/sve: Context switch the SVE registers

Dave Martin
2019-04-25T13:35:59Z

On Wed, Apr 24, 2019 at 03:51:32PM +0100, Alex Bennée wrote:
>
> Dave Martin <Dave.Martin@arm.com> writes:
>
> > On Thu, Apr 04, 2019 at 10:35:02AM +0200, Andrew Jones wrote:
> >> On Thu, Apr 04, 2019 at 09:10:08AM +0100, Dave Martin wrote:
> >> > On Wed, Apr 03, 2019 at 10:01:45PM +0200, Andrew Jones wrote:
> >> > > On Fri, Mar 29, 2019 at 01:00:38PM +0000, Dave Martin wrote:
> >> > > > In order to give each vcpu its own view of the SVE registers, this
> >> > > > patch adds context storage via a new sve_state pointer in struct
> >> > > > vcpu_arch. An additional member sve_max_vl is also added for each
> >> > > > vcpu, to determine the maximum vector length visible to the guest
> >> > > > and thus the value to be configured in ZCR_EL2.LEN while the vcpu
> >> > > > is active. This also determines the layout and size of the storage
> >> > > > in sve_state, which is read and written by the same backend
> >> > > > functions that are used for context-switching the SVE state for
> >> > > > host tasks.
> >> > > >
> >> > > > On SVE-enabled vcpus, SVE access traps are now handled by switching
> >> > > > in the vcpu's SVE context and disabling the trap before returning
> >> > > > to the guest. On other vcpus, the trap is not handled and an exit
> >> > > > back to the host occurs, where the handle_sve() fallback path
> >> > > > reflects an undefined instruction exception back to the guest,
> >> > > > consistently with the behaviour of non-SVE-capable hardware (as was
> >> > > > done unconditionally prior to this patch).
> >> > > >
> >> > > > No SVE handling is added on non-VHE-only paths, since VHE is an
> >> > > > architectural and Kconfig prerequisite of SVE.
> >> > > >
> >> > > > Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> >> > > > Reviewed-by: Julien Thierry <julien.thierry@arm.com>
> >> > > > Tested-by: zhang.lei <zhang.lei@jp.fujitsu.com>
> >> > > >
> >> > > > ---
> >> > > >
> >> > > > Changes since v5:
> >> > > >
> >> > > > * [Julien Thierry, Julien Grall] Commit message typo fixes
> >> > > >
> >> > > > * [Mark Rutland] Rename trap_class to hsr_ec, for consistency with
> >> > > > existing code.
> >> > > >
> >> > > > * [Mark Rutland] Simplify condition for refusing to handle an
> >> > > > FPSIMD/SVE trap, using multiple if () statements for clarity. The
> >> > > > previous condition was a bit tortuous, and how that the static_key
> >> > > > checks have been hoisted out, it makes little difference to the
> >> > > > compiler how we express the condition here.
> >> > > > ---
> >> > > > arch/arm64/include/asm/kvm_host.h | 6 ++++
> >> > > > arch/arm64/kvm/fpsimd.c | 5 +--
> >> > > > arch/arm64/kvm/hyp/switch.c | 75 +++++++++++++++++++++++++++++----------
> >> > > > 3 files changed, 66 insertions(+), 20 deletions(-)
> >> > > >
> >> > > > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> >> > > > index 22cf484..4fabfd2 100644
> >> > > > --- a/arch/arm64/include/asm/kvm_host.h
> >> > > > +++ b/arch/arm64/include/asm/kvm_host.h
> >> > > > @@ -228,6 +228,8 @@ struct vcpu_reset_state {
> >> > > >
> >> > > > struct kvm_vcpu_arch {
> >> > > > struct kvm_cpu_context ctxt;
> >> > > > + void *sve_state;
> >> > > > + unsigned int sve_max_vl;
> >> > > >
> >> > > > /* HYP configuration */
> >> > > > u64 hcr_el2;
> >> > > > @@ -323,6 +325,10 @@ struct kvm_vcpu_arch {
> >> > > > bool sysregs_loaded_on_cpu;
> >> > > > };
> >> > > >
> >> > > > +/* Pointer to the vcpu's SVE FFR for sve_{save,load}_state() */
> >> > > > +#define vcpu_sve_pffr(vcpu) ((void *)((char *)((vcpu)->arch.sve_state) + \
> >> > > > + sve_ffr_offset((vcpu)->arch.sve_max_vl)))
> >> > >
> >> > > Maybe an inline function instead?
> >> >
> >> > I tried, but that requires the definition of struct kvm_vcpu to be
> >> > visible. I failed to get that here without circular #include problems,
> >> > and it looked tricky to fix.
> >>
> >> Ah, OK
> >>
> >> >
> >> > Since this is a small bit of code which is unlikely to get used by
> >> > accident, I decided it was OK to keep it as a macro.
> >> >
> >> > Can you see another way around this?
> >>
> >> Nope
> >
> > OK. If someone eventually solves this, I'd be happy to change to an
> > inline function.
>
> Is the function intended to be used by more call sites? Currently in the
> tree with this plus the v2 fixups I can only see:
>
> arch/arm64/include/asm/kvm_host.h:333:#define vcpu_sve_pffr(vcpu) ((void *)((char *)((vcpu)->arch.sve_state) + \
> arch/arm64/kvm/hyp/switch.c:388: sve_load_state(vcpu_sve_pffr(vcpu),

Probably not, although it was probably used to save the state back
before things were refactored so that fpsimd_save() in
arch/arm64/kernel/fpsimd.c is used instead of separate code to save the
vcpu state.

The expression is ugly so it's nice to abstract it. This also keeps
the sve_load_state() call feeling consistent to the equivalent call in
task_fpsimd_load() in arm64/kernel/fpsimd.c

Other than that, there's no underlying reason for having a macro.

Cheers
---Dave