Re: [PATCH v2 08/18] arm64: KVM: add support to save/restore SPE profiling buffer controls

Andrew Murray
2019-12-24T15:17:44Z

On Tue, Dec 24, 2019 at 10:49:30AM +0000, Andrew Murray wrote:
> On Sat, Dec 21, 2019 at 01:57:55PM +0000, Marc Zyngier wrote:
> > On Fri, 20 Dec 2019 14:30:15 +0000
> > Andrew Murray <andrew.murray@arm.com> wrote:
> >
> > > From: Sudeep Holla <sudeep.holla@arm.com>
> > >
> > > Currently since we don't support profiling using SPE in the guests,
> > > we just save the PMSCR_EL1, flush the profiling buffers and disable
> > > sampling. However in order to support simultaneous sampling both in
> >
> > Is the sampling actually simultaneous? I don't believe so (the whole
> > series would be much simpler if it was).
>
> No the SPE is used by either the guest or host at any one time. I guess
> the term simultaneous was used to refer to illusion given to both guest
> and host that they are able to use it whenever they like. I'll update
> the commit message to drop the magic.
>
>
> >
> > > the host and guests, we need to save and reatore the complete SPE
> >
> > s/reatore/restore/
>
> Noted.
>
>
> >
> > > profiling buffer controls' context.
> > >
> > > Let's add the support for the same and keep it disabled for now.
> > > We can enable it conditionally only if guests are allowed to use
> > > SPE.
> > >
> > > Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
> > > [ Clear PMBSR bit when saving state to prevent spurious interrupts ]
> > > Signed-off-by: Andrew Murray <andrew.murray@arm.com>
> > > ---
> > > arch/arm64/kvm/hyp/debug-sr.c | 51 +++++++++++++++++++++++++++++------
> > > 1 file changed, 43 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/arm64/kvm/hyp/debug-sr.c b/arch/arm64/kvm/hyp/debug-sr.c
> > > index 8a70a493345e..12429b212a3a 100644
> > > --- a/arch/arm64/kvm/hyp/debug-sr.c
> > > +++ b/arch/arm64/kvm/hyp/debug-sr.c
> > > @@ -85,7 +85,8 @@
> > > default: write_debug(ptr[0], reg, 0); \
> > > }
> > >
> > > -static void __hyp_text __debug_save_spe_nvhe(struct kvm_cpu_context *ctxt)
> > > +static void __hyp_text
> > > +__debug_save_spe_nvhe(struct kvm_cpu_context *ctxt, bool full_ctxt)
> >
> > nit: don't split lines like this if you can avoid it. You can put the
> > full_ctxt parameter on a separate line instead.
>
> Yes understood.
>
>
> >
> > > {
> > > u64 reg;
> > >
> > > @@ -102,22 +103,46 @@ static void __hyp_text __debug_save_spe_nvhe(struct kvm_cpu_context *ctxt)
> > > if (reg & BIT(SYS_PMBIDR_EL1_P_SHIFT))
> > > return;
> > >
> > > - /* No; is the host actually using the thing? */
> > > - reg = read_sysreg_s(SYS_PMBLIMITR_EL1);
> > > - if (!(reg & BIT(SYS_PMBLIMITR_EL1_E_SHIFT)))
> > > + /* Save the control register and disable data generation */
> > > + ctxt->sys_regs[PMSCR_EL1] = read_sysreg_el1(SYS_PMSCR);
> > > +
> > > + if (!ctxt->sys_regs[PMSCR_EL1])
> >
> > Shouldn't you check the enable bits instead of relying on the whole
> > thing being zero?
>
> Yes that would make more sense (E1SPE and E0SPE).
>
> I feel that this check makes an assumption about the guest/host SPE
> driver... What happens if the SPE driver writes to some SPE registers
> but doesn't enable PMSCR? If the guest is also using SPE then those
> writes will be lost, when the host returns and the SPE driver enables
> SPE it won't work.
>
> With a quick look at the SPE driver I'm not sure this will happen, but
> even so it makes me nervous relying on these assumptions. I wonder if
> this risk is present in other devices?

In fact, this may be a good reason to trap the SPE registers - this would
allow you to conditionally save/restore based on a dirty bit. It would
also allow you to re-evaluate the SPE interrupt (for example when the guest
clears the status register) and thus potentially reduce any black hole.

Thanks,

Andrew Murray

>
>
> >
> > > return;
> > >
> > > /* Yes; save the control register and disable data generation */
> > > - ctxt->sys_regs[PMSCR_EL1] = read_sysreg_el1(SYS_PMSCR);
> >
> > You've already saved the control register...
>
> I'll remove that.
>
>
> >
> > > write_sysreg_el1(0, SYS_PMSCR);
> > > isb();
> > >
> > > /* Now drain all buffered data to memory */
> > > psb_csync();
> > > dsb(nsh);
> > > +
> > > + if (!full_ctxt)
> > > + return;
> > > +
> > > + ctxt->sys_regs[PMBLIMITR_EL1] = read_sysreg_s(SYS_PMBLIMITR_EL1);
> > > + write_sysreg_s(0, SYS_PMBLIMITR_EL1);
> > > +
> > > + /*
> > > + * As PMBSR is conditionally restored when returning to the host we
> > > + * must ensure the service bit is unset here to prevent a spurious
> > > + * host SPE interrupt from being raised.
> > > + */
> > > + ctxt->sys_regs[PMBSR_EL1] = read_sysreg_s(SYS_PMBSR_EL1);
> > > + write_sysreg_s(0, SYS_PMBSR_EL1);
> > > +
> > > + isb();
> > > +
> > > + ctxt->sys_regs[PMSICR_EL1] = read_sysreg_s(SYS_PMSICR_EL1);
> > > + ctxt->sys_regs[PMSIRR_EL1] = read_sysreg_s(SYS_PMSIRR_EL1);
> > > + ctxt->sys_regs[PMSFCR_EL1] = read_sysreg_s(SYS_PMSFCR_EL1);
> > > + ctxt->sys_regs[PMSEVFR_EL1] = read_sysreg_s(SYS_PMSEVFR_EL1);
> > > + ctxt->sys_regs[PMSLATFR_EL1] = read_sysreg_s(SYS_PMSLATFR_EL1);
> > > + ctxt->sys_regs[PMBPTR_EL1] = read_sysreg_s(SYS_PMBPTR_EL1);
> > > }
> > >
> > > -static void __hyp_text __debug_restore_spe_nvhe(struct kvm_cpu_context *ctxt)
> > > +static void __hyp_text
> > > +__debug_restore_spe_nvhe(struct kvm_cpu_context *ctxt, bool full_ctxt)
> > > {
> > > if (!ctxt->sys_regs[PMSCR_EL1])
> > > return;
> > > @@ -126,6 +151,16 @@ static void __hyp_text __debug_restore_spe_nvhe(struct kvm_cpu_context *ctxt)
> > > isb();
> > >
> > > /* Re-enable data generation */
> > > + if (full_ctxt) {
> > > + write_sysreg_s(ctxt->sys_regs[PMBPTR_EL1], SYS_PMBPTR_EL1);
> > > + write_sysreg_s(ctxt->sys_regs[PMBLIMITR_EL1], SYS_PMBLIMITR_EL1);
> > > + write_sysreg_s(ctxt->sys_regs[PMSFCR_EL1], SYS_PMSFCR_EL1);
> > > + write_sysreg_s(ctxt->sys_regs[PMSEVFR_EL1], SYS_PMSEVFR_EL1);
> > > + write_sysreg_s(ctxt->sys_regs[PMSLATFR_EL1], SYS_PMSLATFR_EL1);
> > > + write_sysreg_s(ctxt->sys_regs[PMSIRR_EL1], SYS_PMSIRR_EL1);
> > > + write_sysreg_s(ctxt->sys_regs[PMSICR_EL1], SYS_PMSICR_EL1);
> > > + write_sysreg_s(ctxt->sys_regs[PMBSR_EL1], SYS_PMBSR_EL1);
> > > + }
> > > write_sysreg_el1(ctxt->sys_regs[PMSCR_EL1], SYS_PMSCR);
> > > }
> > >
> > > @@ -198,7 +233,7 @@ void __hyp_text __debug_restore_host_context(struct kvm_vcpu *vcpu)
> > > guest_ctxt = &vcpu->arch.ctxt;
> > >
> > > if (!has_vhe())
> > > - __debug_restore_spe_nvhe(host_ctxt);
> > > + __debug_restore_spe_nvhe(host_ctxt, false);
> > >
> > > if (!(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY))
> > > return;
> > > @@ -222,7 +257,7 @@ void __hyp_text __debug_save_host_context(struct kvm_vcpu *vcpu)
> > >
> > > host_ctxt = kern_hyp_va(vcpu->arch.host_cpu_context);
> > > if (!has_vhe())
> > > - __debug_save_spe_nvhe(host_ctxt);
> > > + __debug_save_spe_nvhe(host_ctxt, false);
> > > }
> > >
> > > void __hyp_text __debug_save_guest_context(struct kvm_vcpu *vcpu)
> >
> > So all of this is for non-VHE. What happens in the VHE case?
>
> By the end of the series this ends up in __debug_save_host_context which is
> called for both VHE/nVHE - on the re-spin I'll make it not look so confusing.
>
> Thanks,
>
> Andrew Murray
>
> >
> > M.
> > --
> > Jazz is not dead. It just smells funny...
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