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

[Liu ping fan <kernelfans@gmail.com>]

On Fri, Nov 8, 2013 at 11:10 AM, Alexander Graf <agraf@suse.de> wrote:
>
> On 08.11.2013, at 03:44, Liu Ping Fan <kernelfans@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.
>
Maybe I misunderstood the ISA spec, but refer for "6.5.14 System Call
Interrupt", no description about the MSR_PR when sc trigger a syscall
interrupt. So I think, guest application "sc 0" will also fall to the
kernel who owns hypervisor mode.  Am I right?

> 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 :).
>
Sorry, but just want to make clear about the idiom:  0 -> kernel run
with NV, and 1st -> kernel run on HV-KVM and provide PR-KVM to up
layer? Right?

When you say "try to not bounce to the 1st hypervisor ", what is the
exact meaning and how can we achieve this?  I am a quite newer on
powerpc, and hope that I can get more clear figure about it  :)

Thanks

Pingfan
>
> Alex
>
>>
>> Signed-off-by: Liu Ping Fan <pingfank@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
>>
>