On Fri, 2013-11-08 at 04:10 +0100, Alexander Graf wrote:
> On 08.11.2013, at 03:44, Liu Ping Fan <kernelfans@xxxxxxxxx> 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 :).
Well, so unfortunately openstack CI uses PR inside HV pretty
heavily .... it *might* be worthwhile optimizing that path if the patch
is simple enough... I'd make that Paul's call.
Cheers,
Ben.
>
> Alex
>
> >
> > Signed-off-by: Liu Ping Fan <pingfank@xxxxxxxxxxxxxxxxxx>
> > ---
> > 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
> >
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