[PATCH] powerpc: kvm: optimize "sc 0" as fast return

[Alexander Graf]

On 08.11.2013, at 03:44, Liu Ping Fan <kernelfans at gmail.com> wrote:

> syscall is a very common behavior inside guest, and this patch
> optimizes the path for the emulation of BOOK3S_INTERRUPT_SYSCALL,
> so hypervisor can return to guest without heavy exit, i.e, no need
> to swap TLB, HTAB,.. etc

The syscall exit you touch here only happens when you do an sc > 0 with MSR_PR set inside the guest. The only case you realistically see this is when you run PR KVM inside of an HV KVM guest.

I don't think we should optimize for that case. Instead, we should rather try to not bounce to the 1st hypervisor in the first place in that scenario :).


Alex

> 
> Signed-off-by: Liu Ping Fan <pingfank at linux.vnet.ibm.com>
> ---
> Compiled, but lack of bare metal, I have not tested it yet.
> ---
> arch/powerpc/kvm/book3s_hv.c            |  6 ------
> arch/powerpc/kvm/book3s_hv_rmhandlers.S | 13 ++++++++++++-
> 2 files changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 62a2b5a..73dc852 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -628,12 +628,6 @@ static int kvmppc_handle_exit(struct kvm_run *run, struct kvm_vcpu *vcpu,
> 		/* hcall - punt to userspace */
> 		int i;
> 
> -		if (vcpu->arch.shregs.msr & MSR_PR) {
> -			/* sc 1 from userspace - reflect to guest syscall */
> -			kvmppc_book3s_queue_irqprio(vcpu, BOOK3S_INTERRUPT_SYSCALL);
> -			r = RESUME_GUEST;
> -			break;
> -		}
> 		run->papr_hcall.nr = kvmppc_get_gpr(vcpu, 3);
> 		for (i = 0; i < 9; ++i)
> 			run->papr_hcall.args[i] = kvmppc_get_gpr(vcpu, 4 + i);
> diff --git a/arch/powerpc/kvm/book3s_hv_rmhandlers.S b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> index c71103b..9f626c3 100644
> --- a/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> +++ b/arch/powerpc/kvm/book3s_hv_rmhandlers.S
> @@ -1388,7 +1388,8 @@ kvmppc_hisi:
> hcall_try_real_mode:
> 	ld	r3,VCPU_GPR(R3)(r9)
> 	andi.	r0,r11,MSR_PR
> -	bne	guest_exit_cont
> +	/* sc 1 from userspace - reflect to guest syscall */
> +	bne	sc_0_fast_return
> 	clrrdi	r3,r3,2
> 	cmpldi	r3,hcall_real_table_end - hcall_real_table
> 	bge	guest_exit_cont
> @@ -1409,6 +1410,16 @@ hcall_try_real_mode:
> 	ld	r11,VCPU_MSR(r4)
> 	b	fast_guest_return
> 
> +sc_0_fast_return:
> +	ld	r10,VCPU_PC(r9)
> +	ld	r11,VCPU_MSR(r9)
> +	mtspr	SPRN_SRR0,r10
> +	mtspr	SPRN_SRR1,r11
> +	li	r10, BOOK3S_INTERRUPT_SYSCALL
> +	LOAD_REG_IMMEDIATE(r3,0xffffffff87a0ffff)	/* zero 33:36,42:47 */
> +	and	r11,r11,r3
> +	b	fast_guest_return
> +
> 	/* We've attempted a real mode hcall, but it's punted it back
> 	 * to userspace.  We need to restore some clobbered volatiles
> 	 * before resuming the pass-it-to-qemu path */
> -- 
> 1.8.1.4
>