On Tue, Apr 10, 2018 at 05:29:51PM +0200, Christoffer Dall wrote:
> On Tue, Apr 10, 2018 at 11:32:50AM +0100, Dave Martin wrote:
> > On Mon, Apr 09, 2018 at 11:22:43PM +0200, Christoffer Dall wrote:
> > > Hi Dave,
> > >
> > > On Mon, Apr 09, 2018 at 11:53:02AM +0100, Dave Martin wrote:
> > > > This patch refactors KVM to align the host and guest FPSIMD
> > > > save/restore logic with each other for arm64. This reduces the
> > > > number of redundant save/restore operations that must occur, and
> > > > reduces the common-case IRQ blackout time during guest exit storms
> > > > by saving the host state lazily and optimising away the need to
> > > > restore the host state before returning to the run loop.
> > > >
> > >
> > > [...]
> > >
> > > > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> > > > index db08a54..74c5a46 100644
> > > > --- a/arch/arm64/kernel/fpsimd.c
> > > > +++ b/arch/arm64/kernel/fpsimd.c
> > >
> > > [...]
> > >
> > > > @@ -1054,15 +1066,20 @@ void fpsimd_update_current_state(struct user_fpsimd_state const *state)
> > > > local_bh_enable();
> > > > }
> > > >
> > > > +void fpsimd_flush_state(unsigned int *cpu)
> > >
> > > This API looks strange to me, and doesn't seem to be called from
> > > elsewhere. Wouldn't it be more clear if it took a struct thread_struct
> > > pointer instead, or if the logic remained embedded in
> > > fpsimd_flush_task_state ?
> >
> > Hmmm, thanks for spotting this -- it's a throwback to my previous
> > approach.
> >
> > I had intended to align KVM fully with the way host tasks' context is
> > tracked, and this would involve a "most recent cpu FPSIMD loaded on"
> > field in struct vcpu_arch: for ABI reasons this can't easily be tacked
> > onto the end of struct user_fpsimd_state, so it would be necessary for
> > it to be a separate field and passed to the relevant maintenance
> > functions as a separate parameter.
> >
> > This approach would allow the vcpu FPSIMD state to remain in the regs
> > across a context switch without the need to reload it, but this also
> > means that some flushing/invalidation of this cached view of the state
> > would be needed around KVM_GET_ONE_REG etc. and at vcpu destruction
> > time. This function would be part of such a maintenance API.
> >
> > For now though, this seemed like extra complexity for dubious benefit.
> >
> > Unless you think it's worth pursuing this optimisation I should
> > probably get rid of this function. We can always bring this back
> > later if we choose.
> >
>
> Agreed, not need to pursue further optimizations at this time (ie.
> before we have data that indicates it's worth it).
>
>
> > > > +{
> > > > + *cpu = NR_CPUS;
> > > > +}
> > > > +
> > > > /*
> > > > * Invalidate live CPU copies of task t's FPSIMD state
> > > > */
> > > > void fpsimd_flush_task_state(struct task_struct *t)
> > > > {
> > > > - t->thread.fpsimd_cpu = NR_CPUS;
> > > > + fpsimd_flush_state(&t->thread.fpsimd_cpu);
> > > > }
> > > >
> > > > -static inline void fpsimd_flush_cpu_state(void)
> > > > +void fpsimd_flush_cpu_state(void)
> > > > {
> > > > __this_cpu_write(fpsimd_last_state.st, NULL);
> > > > }
> > >
> > > [...]
> > >
> > > > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> > > > index 8605e04..797b259 100644
> > > > --- a/arch/arm64/kvm/hyp/switch.c
> > > > +++ b/arch/arm64/kvm/hyp/switch.c
> > > > @@ -27,6 +27,7 @@
> > > > #include <asm/kvm_mmu.h>
> > > > #include <asm/fpsimd.h>
> > > > #include <asm/debug-monitors.h>
> > > > +#include <asm/thread_info.h>
> > > >
> > > > static bool __hyp_text __fpsimd_enabled_nvhe(void)
> > > > {
> > > > @@ -47,24 +48,40 @@ bool __hyp_text __fpsimd_enabled(void)
> > > > return __fpsimd_is_enabled()();
> > > > }
> > > >
> > > > -static void __hyp_text __activate_traps_vhe(void)
> > > > +static bool update_fp_enabled(struct kvm_vcpu *vcpu)
> > >
> > > I think this needs a __hyp_text in the unlikely case that this function
> > > is not inlined in the _nvhe caller by the compiler.
> >
> > You're right. I'll add it.
> >
> > > > +{
> > > > + if (vcpu->arch.host_thread_info->flags & _TIF_FOREIGN_FPSTATE) {
> > > > + vcpu->arch.host_fpsimd_state = NULL;
> > > > + vcpu->arch.fp_enabled = false;
> > > > + }
> > >
> > > I'm not clear why the above logic can't go into kvm_arch_vcpu_load_fp
> > > and why we can't simply check TIF_FOREIGN_FPSTATE in __hyp_switch_fpsimd
> > > instead?
> >
> > The situation can change in between _load_fp() and here, because of
> > kernel-mode NEON.
> >
> > Also, we can't defer this check to __hyp_switch_fpsimd() because this is
> > the logic for deciding whether to re-enable the Hyp FPSIMD trap in the
> > first place.
> >
> >
> > Here's a scenario:
> >
> > * We're on a second iteration of the run loop, with the vcpu state loaded:
> > * fp_enabled = true, TIF_FOREIGN_FPSTATE is clear,
> > executing in the host with irqs enabled.
> >
> > * A softirq uses kernel-mode NEON:
> > * vcpu FPSIMD state is saved back to memory
> > * TIF_FOREIGN_FPSTATE now set
> > * CPU FPSIMD regs now contain garbage
> >
> > * local_irq_disable(), and enter guest
> >
> > * fp_enabled == true, but out of date:
> > * update_fp_enabled detects this condition by observing that
> > TIF_FOREIGN_FPSTATE is set and clearing fp_enabled.
> > * the (updated) value of fp_enabled determines that the FPSIMD trap
> > should be enabled
> >
> > * __hyp_switch_fpsimd() saves no host state (because it was already
> > saved and anyway host_fpsimd_state is NULL)
> > * __hyp_switch_fpsimd() loads the guest state
> >
> >
> > Is there a way to simplify the code that doesn't break this?
> >
>
> Hmmm, maybe not. At first glance I thought that TIF_FOREIGN_FPSTATE was
> tied to the host_fpsimd_state being NULL or not, but it appears we can
> have host_fpsimd_state be NULL while still not have TIF_FOREIGN_FPSTATE.
>
> That in turn means that we have three boolean values to decribe our
> state:
>
> TIF_FOREIGN_PSTATE | host_fpsimd_state | fp_enabled | VFP Regs
> -----------------------------------------------------------------------
> 0 | 0 | 0 | Not allowed?
> 0 | 0 | 1 | vcpu state
> 0 | 1 | 0 | user state
> 0 | 1 | 1 | Not allowed?
> 1 | x | x | Garbage
>
> If I got this vaguely correct, then indeed there doesn't seem to be any
> way to simplify this.
Basically yes, I think.
This way of summarising it makes it look like we don't really need three
variables and maybe that's the case. The current code may be a bit
clearer than the alternative though.
I've preferred to keep TIF_FOREIGN_FPSTATE written by the host only and
fp_enabled written by the guest only to demarcate responsibility a bit,
but this does mean that these flags have overlapping meanings and lag
one another some of the time.
> > >
> > > > +
> > > > + return vcpu->arch.fp_enabled;
> > > > +}
> > > > +
> > > > +static void __hyp_text __activate_traps_vhe(struct kvm_vcpu *vcpu)
> > > > {
> > > > u64 val;
> > > >
> > > > val = read_sysreg(cpacr_el1);
> > > > val |= CPACR_EL1_TTA;
> > > > - val &= ~(CPACR_EL1_FPEN | CPACR_EL1_ZEN);
> > > > + val &= ~CPACR_EL1_ZEN;
> > > > + if (!update_fp_enabled(vcpu))
> > > > + val &= ~CPACR_EL1_FPEN;
> > > > +
> > > > write_sysreg(val, cpacr_el1);
> > > >
> > > > write_sysreg(kvm_get_hyp_vector(), vbar_el1);
> > > > }
> > > >
> > > > -static void __hyp_text __activate_traps_nvhe(void)
> > > > +static void __hyp_text __activate_traps_nvhe(struct kvm_vcpu *vcpu)
> > > > {
> > > > u64 val;
> > > >
> > > > val = CPTR_EL2_DEFAULT;
> > > > - val |= CPTR_EL2_TTA | CPTR_EL2_TFP | CPTR_EL2_TZ;
> > > > + val |= CPTR_EL2_TTA | CPTR_EL2_TZ;
> > > > + if (!update_fp_enabled(vcpu))
> > > > + val |= CPTR_EL2_TFP;
> > > > +
> > > > write_sysreg(val, cptr_el2);
> > > > }
> > > >
> > >
> > > [...]
> > >
> > > Otherwise this approach looks quite good to me overall. Are you
> > > planning to add SVE support before removing the RFC from this series?
> >
> > Yes :)
> >
> > (I've been delaying that while we get the basic approach sorted out.)
> >
>
> Makes sense, was just trying to understand if this could somehow
> actually work without adding SVE support.
We could temprorarily disable CONFIG_ARM64_SVE, but that would be a
bodge. I don't think the necessary changes for supporting host SVE
will be complex, so I would prefer not to merge without them. I don't
anticipate it taking more than a day or two to post an update to the
series.
Cheers
---Dave
>
> Thanks,
> -Christoffer
>
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