Hi Tomasz,
On Wed, Jan 31, 2018 at 01:17:36PM +0100, Tomasz Nowicki wrote:
> On 12.01.2018 13:07, Christoffer Dall wrote:
> >There is no need to enable/disable traps to FP registers on every switch
> >to/from the VM, because the host kernel does not use this resource
> >without calling vcpu_put. We can therefore move things around enough
> >that we still always write FPEXC32_EL2 before programming CPTR_EL2 but
> >only program these during vcpu load/put.
> >
> >Signed-off-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
> >---
> > arch/arm64/include/asm/kvm_hyp.h | 6 +++++
> > arch/arm64/kvm/hyp/switch.c | 51 +++++++++++++++++++++++++++++-----------
> > arch/arm64/kvm/hyp/sysreg-sr.c | 12 ++++++++--
> > 3 files changed, 53 insertions(+), 16 deletions(-)
> >
> >diff --git a/arch/arm64/include/asm/kvm_hyp.h b/arch/arm64/include/asm/kvm_hyp.h
> >index 3f54c55f77a1..ffd62e31f134 100644
> >--- a/arch/arm64/include/asm/kvm_hyp.h
> >+++ b/arch/arm64/include/asm/kvm_hyp.h
> >@@ -148,6 +148,12 @@ void __fpsimd_save_state(struct user_fpsimd_state *fp_regs);
> > void __fpsimd_restore_state(struct user_fpsimd_state *fp_regs);
> > bool __fpsimd_enabled(void);
> >+void __activate_traps_nvhe_load(struct kvm_vcpu *vcpu);
> >+void __deactivate_traps_nvhe_put(void);
> >+
> >+void activate_traps_vhe_load(struct kvm_vcpu *vcpu);
> >+void deactivate_traps_vhe_put(void);
> >+
> > u64 __guest_enter(struct kvm_vcpu *vcpu, struct kvm_cpu_context *host_ctxt);
> > void __noreturn __hyp_do_panic(unsigned long, ...);
> >diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> >index c01bcfc3fb52..d14ab9650f81 100644
> >--- a/arch/arm64/kvm/hyp/switch.c
> >+++ b/arch/arm64/kvm/hyp/switch.c
> >@@ -24,22 +24,25 @@
> > #include <asm/fpsimd.h>
> > #include <asm/debug-monitors.h>
> >-static void __hyp_text __activate_traps_common(struct kvm_vcpu *vcpu)
> >+static void __hyp_text __activate_traps_fpsimd32(struct kvm_vcpu *vcpu)
> > {
> > /*
> >- * We are about to set CPTR_EL2.TFP to trap all floating point
> >- * register accesses to EL2, however, the ARM ARM clearly states that
> >- * traps are only taken to EL2 if the operation would not otherwise
> >- * trap to EL1. Therefore, always make sure that for 32-bit guests,
> >- * we set FPEXC.EN to prevent traps to EL1, when setting the TFP bit.
> >- * If FP/ASIMD is not implemented, FPEXC is UNDEFINED and any access to
> >- * it will cause an exception.
> >+ * We are about to trap all floating point register accesses to EL2,
> >+ * however, traps are only taken to EL2 if the operation would not
> >+ * otherwise trap to EL1. Therefore, always make sure that for 32-bit
> >+ * guests, we set FPEXC.EN to prevent traps to EL1, when setting the
> >+ * TFP bit. If FP/ASIMD is not implemented, FPEXC is UNDEFINED and
> >+ * any access to it will cause an exception.
> > */
> > if (vcpu_el1_is_32bit(vcpu) && system_supports_fpsimd() &&
> > !vcpu->arch.guest_vfp_loaded) {
> > write_sysreg(1 << 30, fpexc32_el2);
> > isb();
> > }
> >+}
> >+
> >+static void __hyp_text __activate_traps_common(struct kvm_vcpu *vcpu)
> >+{
> > write_sysreg(vcpu->arch.hcr_el2, hcr_el2);
> > /* Trap on AArch32 cp15 c15 (impdef sysregs) accesses (EL1 or EL0) */
> >@@ -61,10 +64,12 @@ static void __hyp_text __deactivate_traps_common(void)
> > write_sysreg(0, pmuserenr_el0);
> > }
> >-static void __hyp_text __activate_traps_vhe(struct kvm_vcpu *vcpu)
> >+void activate_traps_vhe_load(struct kvm_vcpu *vcpu)
> > {
> > u64 val;
> >+ __activate_traps_fpsimd32(vcpu);
> >+
> > val = read_sysreg(cpacr_el1);
> > val |= CPACR_EL1_TTA;
> > val &= ~CPACR_EL1_ZEN;
> >@@ -73,14 +78,26 @@ static void __hyp_text __activate_traps_vhe(struct kvm_vcpu *vcpu)
> > else
> > val &= ~CPACR_EL1_FPEN;
> > write_sysreg(val, cpacr_el1);
>
> Giving that you move this code to kvm_vcpu_load_sysregs() I am wondering if
> we have to deactivate FPEN trap here. IIUC, we call
> kvm_vcpu_load_sysregs()->activate_traps_vhe_load() and then
> kvm_vcpu_put_sysregs() by design. So vcpu->arch.guest_vfp_loaded should be
> always 0 here since it is zeroed in kvm_vcpu_put_sysregs(). The same for
> nvhe case below.
>
You're absolutely right, we can enable the trapping unconditionally on
this path.
Thanks,
-Christoffer
