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

[Andrew Murray <andrew.murray@arm.com>]

On Fri, Jan 10, 2020 at 11:51:39AM +0000, Marc Zyngier wrote:
> 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@arm.com> wrote:
> > > >
> > > > [somehow managed not to do a reply all, re-sending]
> > > >
> > > > > From: Sudeep Holla <sudeep.holla@arm.com>
> > > > >
> > > > > 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@arm.com>
> > > > > Signed-off-by: Andrew Murray <andrew.murray@arm.com>
> > > > > ---
> > > > >  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.

Yes that makes sense.


> 
> >    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.

That make me feel better :)

Thanks,

Andrew Murray

> 
>         M.
> -- 
> Jazz is not dead. It just smells funny...
_______________________________________________
kvmarm mailing list
kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm