On Thu, Nov 05, 2015 at 04:23:41PM -0800, Mario Smarduch wrote:
>
>
> On 11/5/2015 6:48 AM, Christoffer Dall wrote:
> > On Fri, Oct 30, 2015 at 02:56:32PM -0700, Mario Smarduch wrote:
> >> This patch tracks vfp/simd hardware state with a vcpu lazy flag. vCPU lazy
> >> flag is set on guest access and traps to vfp/simd hardware switch handler. On
> >> vm-enter if lazy flag is set skip trap enable and save host fpexc. On
> >> vm-exit if flag is set skip hardware context switch and return to host with
> >> guest context. In vcpu_put check if vcpu lazy flag is set, and execute a
> >> hardware context switch to restore host.
> >>
> >> Also some arm64 field and empty function are added to compile for arm64.
> >>
> >> Signed-off-by: Mario Smarduch <m.smarduch@xxxxxxxxxxx>
> >> ---
> >> arch/arm/include/asm/kvm_host.h | 1 +
> >> arch/arm/kvm/arm.c | 6 ++++
> >> arch/arm/kvm/interrupts.S | 60 ++++++++++++++++++++++++++++-----------
> >> arch/arm/kvm/interrupts_head.S | 14 +++++----
> >> arch/arm64/include/asm/kvm_host.h | 4 +++
> >> 5 files changed, 63 insertions(+), 22 deletions(-)
> >>
> >> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> >> index f1bf551..a9e86e0 100644
> >> --- a/arch/arm/include/asm/kvm_host.h
> >> +++ b/arch/arm/include/asm/kvm_host.h
> >> @@ -227,6 +227,7 @@ int kvm_perf_teardown(void);
> >> void kvm_mmu_wp_memory_region(struct kvm *kvm, int slot);
> >>
> >> struct kvm_vcpu *kvm_mpidr_to_vcpu(struct kvm *kvm, unsigned long mpidr);
> >> +void kvm_restore_host_vfp_state(struct kvm_vcpu *);
> >>
> >> static inline void kvm_arch_hardware_disable(void) {}
> >> static inline void kvm_arch_hardware_unsetup(void) {}
> >> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> >> index dc017ad..11a56fe 100644
> >> --- a/arch/arm/kvm/arm.c
> >> +++ b/arch/arm/kvm/arm.c
> >> @@ -296,6 +296,12 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> >> void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> >> {
> >> /*
> >> + * If fp/simd registers are dirty save guest, restore host before
> >
> > If the fp/simd registers are dirty, then restore the host state before
> I'd drop 'releasing the cpu', the vcpu thread may be returning to
> user mode.
> >
> >> + * releasing the cpu.
> >> + */
> >> + if (vcpu->arch.vfp_dirty)
> >> + kvm_restore_host_vfp_state(vcpu);
> >> + /*
> >> * The arch-generic KVM code expects the cpu field of a vcpu to be -1
> >> * if the vcpu is no longer assigned to a cpu. This is used for the
> >> * optimized make_all_cpus_request path.
> >> diff --git a/arch/arm/kvm/interrupts.S b/arch/arm/kvm/interrupts.S
> >> index 900ef6d..ca25314 100644
> >> --- a/arch/arm/kvm/interrupts.S
> >> +++ b/arch/arm/kvm/interrupts.S
> >> @@ -28,6 +28,32 @@
> >> #include "interrupts_head.S"
> >>
> >> .text
> >> +/**
> >> + * void kvm_restore_host_vfp_state(struct vcpu *vcpu) - Executes lazy
> >
> > nit: Can you move the multi-line description of the function into a
> > separate paragraph?
> Sure.
> >
> >> + * fp/simd switch, saves the guest, restores host. Called from host
> >> + * mode, placed outside of hyp region start/end.
> >
> > Put the description in a separate paragraph and get rid of the "executes
> > lazy fp/simd swithch" part, that doesn't help understanding. Just say
> > that this funciton restores the host state.
> Sure.
> >
> >> + */
> >> +ENTRY(kvm_restore_host_vfp_state)
> >> +#ifdef CONFIG_VFPv3
> >> + push {r4-r7}
> >> +
> >> + add r7, vcpu, #VCPU_VFP_GUEST
> >> + store_vfp_state r7
> >> +
> >> + add r7, vcpu, #VCPU_VFP_HOST
> >> + ldr r7, [r7]
> >> + restore_vfp_state r7
> >> +
> >> + ldr r3, [vcpu, #VCPU_VFP_HOST_FPEXC]
> >> + VFPFMXR FPEXC, r3
> >> +
> >> + mov r3, #0
> >> + strb r3, [vcpu, #VCPU_VFP_DIRTY]
> >> +
> >> + pop {r4-r7}
> >> +#endif
> >> + bx lr
> >> +ENDPROC(kvm_restore_host_vfp_state)
> >>
> >> __kvm_hyp_code_start:
> >> .globl __kvm_hyp_code_start
> >> @@ -119,11 +145,16 @@ ENTRY(__kvm_vcpu_run)
> >> @ If the host kernel has not been configured with VFPv3 support,
> >> @ then it is safer if we deny guests from using it as well.
> >> #ifdef CONFIG_VFPv3
> >> - @ Set FPEXC_EN so the guest doesn't trap floating point instructions
> >> + @ fp/simd register file has already been accessed, so skip host fpexc
> >> + @ save and access trap enable.
> >> + vfp_inlazy_mode r7, skip_guest_vfp_trap
> >
> > So, why do we need to touch this register at all on every CPU exit?
> >
> > Is it not true that we can only be in one of two state:
> > 1) The register file is not dirty (not touched by the guest) and we
> > should trap
> > 2) The register file is dirty, and we should not trap to EL2?
> >
> > Only in the first case do we need to set the FPEXC, and couldn't we just
> > do that on vcpu_load and git rid of all this? (except HCPTR_TCP which
> > we still need to adjust).
>
> I'm trying to think what happens if you're preempted after you saved
> the FPEXC and set the FPEXC_EN bit in kvm_arch_vcpu_load(). Could some
> thread pick up a bad FPEXC? May be possible to undo in the vcpu_put().
If you're preempted, vcpu_put will be called. See kvm_preempt_ops in
virt/kvm/kvm_main.c.
>
> >
> >> +
> >> VFPFMRX r2, FPEXC @ VMRS
> >> - push {r2}
> >> + str r2, [vcpu, #VCPU_VFP_HOST_FPEXC]
> >> orr r2, r2, #FPEXC_EN
> >> VFPFMXR FPEXC, r2 @ VMSR
> >> + set_hcptr vmentry, (HCPTR_TCP(10) | HCPTR_TCP(11))
> >> +skip_guest_vfp_trap:
> >> #endif
> >>
> >> @ Configure Hyp-role
> >> @@ -131,7 +162,7 @@ ENTRY(__kvm_vcpu_run)
> >>
> >> @ Trap coprocessor CRx accesses
> >> set_hstr vmentry
> >> - set_hcptr vmentry, (HCPTR_TTA | HCPTR_TCP(10) | HCPTR_TCP(11))
> >> + set_hcptr vmentry, (HCPTR_TTA)
> >
> > based on the above I think you can rework this to set the mask based on
> > the dirty flag and only hit the HCPTR once.
>
> Not sure how to do this, tracing always needs to be enabled, and it's
> independent of FP dirty state.
here, you do:
ldr r4, HCPTR_TTA
vfp_skip_if_dirty skip_vfp_trap
orr r4, r4, #(HCPTR_TCP(10) | HCPTR_TCP(11))
skip_vfp_trap:
set_hcptr vmentry, r4
if that works with the necessary rework of set_hcptr to take a register,
if the orr can be encoded propertly etc. Maybe it's not worth it, it
just feels weird to touch this registers twice. Perhaps the nicer fix
is to just rename/refactor set_hcptr to be two functions, set_hcptr_bits
and clear_hcptr_bits.
Thanks,
-Christoffer
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