Re: [PATCH v2 09/18] arm64: KVM: enable conditional save/restore full SPE profiling buffer controls

From: Marc Zyngier
Date: Fri Jan 10 2020 - 06:51:44 EST

On 2020-01-10 11:04, Andrew Murray wrote:
On Fri, Jan 10, 2020 at 10:54:36AM +0000, Andrew Murray wrote:
On Sat, Dec 21, 2019 at 02:13:25PM +0000, Marc Zyngier wrote:
> On Fri, 20 Dec 2019 14:30:16 +0000
> Andrew Murray <andrew.murray@xxxxxxx> wrote:
>
> [somehow managed not to do a reply all, re-sending]
>
> > From: Sudeep Holla <sudeep.holla@xxxxxxx>
> >
> > Now that we can save/restore the full SPE controls, we can enable it
> > if SPE is setup and ready to use in KVM. It's supported in KVM only if
> > all the CPUs in the system supports SPE.
> >
> > However to support heterogenous systems, we need to move the check if
> > host supports SPE and do a partial save/restore.
>
> No. Let's just not go down that path. For now, KVM on heterogeneous
> systems do not get SPE. If SPE has been enabled on a guest and a CPU
> comes up without SPE, this CPU should fail to boot (same as exposing a
> feature to userspace).
>
> >
> > Signed-off-by: Sudeep Holla <sudeep.holla@xxxxxxx>
> > Signed-off-by: Andrew Murray <andrew.murray@xxxxxxx>
> > ---
> > arch/arm64/kvm/hyp/debug-sr.c | 33 ++++++++++++++++-----------------
> > include/kvm/arm_spe.h | 6 ++++++
> > 2 files changed, 22 insertions(+), 17 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/hyp/debug-sr.c b/arch/arm64/kvm/hyp/debug-sr.c
> > index 12429b212a3a..d8d857067e6d 100644
> > --- a/arch/arm64/kvm/hyp/debug-sr.c
> > +++ b/arch/arm64/kvm/hyp/debug-sr.c
> > @@ -86,18 +86,13 @@
> > }
> >
> > static void __hyp_text
> > -__debug_save_spe_nvhe(struct kvm_cpu_context *ctxt, bool full_ctxt)
> > +__debug_save_spe_context(struct kvm_cpu_context *ctxt, bool full_ctxt)
> > {
> > u64 reg;
> >
> > /* Clear pmscr in case of early return */
> > ctxt->sys_regs[PMSCR_EL1] = 0;
> >
> > - /* SPE present on this CPU? */
> > - if (!cpuid_feature_extract_unsigned_field(read_sysreg(id_aa64dfr0_el1),
> > - ID_AA64DFR0_PMSVER_SHIFT))
> > - return;
> > -
> > /* Yes; is it owned by higher EL? */
> > reg = read_sysreg_s(SYS_PMBIDR_EL1);
> > if (reg & BIT(SYS_PMBIDR_EL1_P_SHIFT))
> > @@ -142,7 +137,7 @@ __debug_save_spe_nvhe(struct kvm_cpu_context *ctxt, bool full_ctxt)
> > }
> >
> > static void __hyp_text
> > -__debug_restore_spe_nvhe(struct kvm_cpu_context *ctxt, bool full_ctxt)
> > +__debug_restore_spe_context(struct kvm_cpu_context *ctxt, bool full_ctxt)
> > {
> > if (!ctxt->sys_regs[PMSCR_EL1])
> > return;
> > @@ -210,11 +205,14 @@ void __hyp_text __debug_restore_guest_context(struct kvm_vcpu *vcpu)
> > struct kvm_guest_debug_arch *host_dbg;
> > struct kvm_guest_debug_arch *guest_dbg;
> >
> > + host_ctxt = kern_hyp_va(vcpu->arch.host_cpu_context);
> > + guest_ctxt = &vcpu->arch.ctxt;
> > +
> > + __debug_restore_spe_context(guest_ctxt, kvm_arm_spe_v1_ready(vcpu));
> > +
> > if (!(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY))
> > return;
> >
> > - host_ctxt = kern_hyp_va(vcpu->arch.host_cpu_context);
> > - guest_ctxt = &vcpu->arch.ctxt;
> > host_dbg = &vcpu->arch.host_debug_state.regs;
> > guest_dbg = kern_hyp_va(vcpu->arch.debug_ptr);
> >
> > @@ -232,8 +230,7 @@ void __hyp_text __debug_restore_host_context(struct kvm_vcpu *vcpu)
> > host_ctxt = kern_hyp_va(vcpu->arch.host_cpu_context);
> > guest_ctxt = &vcpu->arch.ctxt;
> >
> > - if (!has_vhe())
> > - __debug_restore_spe_nvhe(host_ctxt, false);
> > + __debug_restore_spe_context(host_ctxt, kvm_arm_spe_v1_ready(vcpu));
>
> So you now do an unconditional save/restore on the exit path for VHE as
> well? Even if the host isn't using the SPE HW? That's not acceptable
> as, in most cases, only the host /or/ the guest will use SPE. Here, you
> put a measurable overhead on each exit.
>
> If the host is not using SPE, then the restore/save should happen in
> vcpu_load/vcpu_put. Only if the host is using SPE should you do
> something in the run loop. Of course, this only applies to VHE and
> non-VHE must switch eagerly.
>

On VHE where SPE is used in the guest only - we save/restore in vcpu_load/put.

On VHE where SPE is used in the host only - we save/restore in the run loop.

On VHE where SPE is used in guest and host - we save/restore in the run loop.

As the guest can't trace EL2 it doesn't matter if we restore guest SPE early
in the vcpu_load/put functions. (I assume it doesn't matter that we restore
an EL0/EL1 profiling buffer address at this point and enable tracing given
that there is nothing to trace until entering the guest).

However the reason for moving save/restore to vcpu_load/put when the host is
using SPE is to minimise the host EL2 black-out window.


On nVHE we always save/restore in the run loop. For the SPE guest-use-only
use-case we can't save/restore in vcpu_load/put - because the guest runs at
the same ELx level as the host - and thus doing so would result in the guest
tracing part of the host.

Though if we determine that (for nVHE systems) the guest SPE is profiling only
EL0 - then we could also save/restore in vcpu_load/put where SPE is only being
used in the guest.

Does that make sense, are my reasons correct?

Also I'm making the following assumptions:

- We determine if the host or guest are using SPE by seeing if profiling
(e.g. PMSCR_EL1) is enabled. That should determine *when* we restore as per
my previous email.

Yes.

- I'm less sure on this: We should determine *what* we restore based on the
availability of the SPE feature and not if it is being used - so for guest
this is if the guest has the feature on the vcpu. For host this is based on
the CPU feature registers.

As long as the guest's feature is conditionned on the HW being present *and*
that you're running on a CPU that has the HW.

The downshot of this is that if you have SPE support present on guest and
host and they aren't being used, then you still save/restore upon entering/
leaving a guest. The reason I feel this is needed is to prevent the issue
where the host starts programming the SPE registers, but is preempted by
KVM entering a guest, before it could enable host SPE. Thus when we enter the
guest we don't save all the registers, we return to the host and the host
SPE carries on from where it left of and enables it - yet because we didn't
restore all the programmed registers it doesn't work.

Saving the host registers is never optional if they are shared with the guest.

M.
--
Jazz is not dead. It just smells funny...