[RFC PATCH v2 2/3] KVM: arm64: Convert lazy FPSIMD context switch trap to C
Dave Martin
Dave.Martin at arm.com
Fri Apr 6 08:51:53 PDT 2018
On Fri, Apr 06, 2018 at 04:25:57PM +0100, Marc Zyngier wrote:
> Hi Dave,
>
> On 06/04/18 16:01, Dave Martin wrote:
> > To make the lazy FPSIMD context switch trap code easier to hack on,
> > this patch converts it to C.
> >
> > This is not amazingly efficient, but the trap should typically only
> > be taken once per host context switch.
> >
> > Signed-off-by: Dave Martin <Dave.Martin at arm.com>
> >
> > ---
> >
> > Since RFCv1:
> >
> > * Fix indentation to be consistent with the rest of the file.
> > * Add missing ! to write back to sp with attempting to push regs.
> > ---
> > arch/arm64/kvm/hyp/entry.S | 57 +++++++++++++++++----------------------------
> > arch/arm64/kvm/hyp/switch.c | 24 +++++++++++++++++++
> > 2 files changed, 46 insertions(+), 35 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> > index fdd1068..47c6a78 100644
> > --- a/arch/arm64/kvm/hyp/entry.S
> > +++ b/arch/arm64/kvm/hyp/entry.S
> > @@ -176,41 +176,28 @@ ENTRY(__fpsimd_guest_restore)
> > // x1: vcpu
> > // x2-x29,lr: vcpu regs
> > // vcpu x0-x1 on the stack
> > - stp x2, x3, [sp, #-16]!
> > - stp x4, lr, [sp, #-16]!
> > -
> > -alternative_if_not ARM64_HAS_VIRT_HOST_EXTN
> > - mrs x2, cptr_el2
> > - bic x2, x2, #CPTR_EL2_TFP
> > - msr cptr_el2, x2
> > -alternative_else
> > - mrs x2, cpacr_el1
> > - orr x2, x2, #CPACR_EL1_FPEN
> > - msr cpacr_el1, x2
> > -alternative_endif
> > - isb
> > -
> > - mov x3, x1
> > -
> > - ldr x0, [x3, #VCPU_HOST_CONTEXT]
> > - kern_hyp_va x0
> > - add x0, x0, #CPU_GP_REG_OFFSET(CPU_FP_REGS)
> > - bl __fpsimd_save_state
> > -
> > - add x2, x3, #VCPU_CONTEXT
> > - add x0, x2, #CPU_GP_REG_OFFSET(CPU_FP_REGS)
> > - bl __fpsimd_restore_state
> > -
> > - // Skip restoring fpexc32 for AArch64 guests
> > - mrs x1, hcr_el2
> > - tbnz x1, #HCR_RW_SHIFT, 1f
> > - ldr x4, [x3, #VCPU_FPEXC32_EL2]
> > - msr fpexc32_el2, x4
> > -1:
> > - ldp x4, lr, [sp], #16
> > - ldp x2, x3, [sp], #16
> > - ldp x0, x1, [sp], #16
> > -
> > + stp x2, x3, [sp, #-144]!
> > + stp x4, x5, [sp, #16]
> > + stp x6, x7, [sp, #32]
> > + stp x8, x9, [sp, #48]
> > + stp x10, x11, [sp, #64]
> > + stp x12, x13, [sp, #80]
> > + stp x14, x15, [sp, #96]
> > + stp x16, x17, [sp, #112]
> > + stp x18, lr, [sp, #128]
> > +
> > + bl __hyp_switch_fpsimd
> > +
> > + ldp x4, x5, [sp, #16]
> > + ldp x6, x7, [sp, #32]
> > + ldp x8, x9, [sp, #48]
> > + ldp x10, x11, [sp, #64]
> > + ldp x12, x13, [sp, #80]
> > + ldp x14, x15, [sp, #96]
> > + ldp x16, x17, [sp, #112]
> > + ldp x18, lr, [sp, #128]
> > + ldp x0, x1, [sp, #144]
> > + ldp x2, x3, [sp], #160
>
> I can't say I'm overly thrilled with adding another save/restore
> sequence. How about treating it like a real guest exit instead? Granted,
> there is a bit more overhead to it, but as you pointed out above, this
> should be pretty rare...
I have no objection to handling this after exiting back to
__kvm_vcpu_run(), provided the performance is deemed acceptable.
> Something like this?
>
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index f6648a3e4152..3c388f5c394f 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -27,6 +27,7 @@
> #define ARM_EXCEPTION_IRQ 0
> #define ARM_EXCEPTION_EL1_SERROR 1
> #define ARM_EXCEPTION_TRAP 2
> +#define ARM_EXCEPTION_FP 3
> /* The hyp-stub will return this for any kvm_call_hyp() call */
> #define ARM_EXCEPTION_HYP_GONE HVC_STUB_ERR
>
> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
> index bffece27b5c1..e32dd00410f8 100644
> --- a/arch/arm64/kvm/hyp/hyp-entry.S
> +++ b/arch/arm64/kvm/hyp/hyp-entry.S
> @@ -129,11 +129,12 @@ el1_trap:
> */
> alternative_if_not ARM64_HAS_NO_FPSIMD
> cmp x0, #ESR_ELx_EC_FP_ASIMD
> - b.eq __fpsimd_guest_restore
> + mov x0, #ARM_EXCEPTION_FP
> + b.eq 1f
> alternative_else_nop_endif
>
> mov x0, #ARM_EXCEPTION_TRAP
> - b __guest_exit
> +1: b __guest_exit
>
> el1_irq:
> get_vcpu_ptr x1, x0
> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index d9645236e474..50b98ac39480 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -325,6 +325,10 @@ static bool __hyp_text __skip_instr(struct kvm_vcpu *vcpu)
> */
> static bool __hyp_text fixup_guest_exit(struct kvm_vcpu *vcpu, u64 *exit_code)
> {
> + if (ARM_EXCEPTION_CODE(*exit_code) == ARM_EXCEPTION_FP) {
> + __hyp_switch_fpsim(read_sysreg_el2(esr), vcpu);
> + return true;
> + }
The esr is a dummy argument here, so we could simply get rid of it.
> if (ARM_EXCEPTION_CODE(*exit_code) != ARM_EXCEPTION_IRQ)
> vcpu->arch.fault.esr_el2 = read_sysreg_el2(esr);
>
>
> > eret
> > ENDPROC(__fpsimd_guest_restore)
> >
> > diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> > index 870f4b1..8605e04 100644
> > --- a/arch/arm64/kvm/hyp/switch.c
> > +++ b/arch/arm64/kvm/hyp/switch.c
> > @@ -440,6 +440,30 @@ int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)
> > return exit_code;
> > }
> >
> > +void __hyp_text __hyp_switch_fpsimd(u64 esr __always_unused,
> > + struct kvm_vcpu *vcpu)
> > +{
> > + kvm_cpu_context_t *host_ctxt;
> > +
> > + if (has_vhe())
> > + write_sysreg(read_sysreg(cpacr_el1) | CPACR_EL1_FPEN,
> > + cpacr_el1);
> > + else
> > + write_sysreg(read_sysreg(cptr_el2) & ~(u64)CPTR_EL2_TFP,
> > + cptr_el2);
> > +
> > + isb();
> > +
> > + host_ctxt = kern_hyp_va(vcpu->arch.host_cpu_context);
> > + __fpsimd_save_state(&host_ctxt->gp_regs.fp_regs);
> > + __fpsimd_restore_state(&vcpu->arch.ctxt.gp_regs.fp_regs);
> > +
> > + /* Skip restoring fpexc32 for AArch64 guests */
> > + if (!(read_sysreg(hcr_el2) & HCR_RW))
> > + write_sysreg(vcpu->arch.ctxt.sys_regs[FPEXC32_EL2],
> > + fpexc32_el2);
> > +}
> > +
[...]
Cheers
---Dave
More information about the linux-arm-kernel
mailing list