Subject: Re: [PATCH] KVM: PPC: Book3S HV: fix handling for interrupted H_ENTER_NESTED

From: Suraj Jitindar Singh <[email-masked]>
Date: Wed, 14 Nov 2018 14:09:03 +1100

On Thu, 2018-11-08 at 21:27 -0600, Michael Roth wrote:
> While running a nested guest VCPU on L0 via H_ENTER_NESTED hcall, a
> pending signal in the L0 QEMU process can generate the following
> sequence:
> 
>   ret0 = kvmppc_pseries_do_hcall()
>     ret1 = kvmhv_enter_nested_guest()
>       ret2 = kvmhv_run_single_vcpu()
>       if (ret2 == -EINTR)
>         return H_INTERRUPT
>     if (ret1 == H_INTERRUPT)
>       kvmppc_set_gpr(vcpu, 3, 0)
>       return -EINTR
>     /* skipped: */
>     kvmppc_set_gpr(vcpu, 3, ret)
>     vcpu->arch.hcall_needed = 0
>     return RESUME_GUEST
> 
> which causes an exit to L0 userspace with ret0 == -EINTR.
> 
> The intention seems to be to set the hcall return value to 0 (via
> VCPU r3) so that L1 will see a successful return from H_ENTER_NESTED
> once we resume executing the VCPU. However, because we don't set
> vcpu->arch.hcall_needed = 0, we do the following once userspace
> resumes execution via kvm_arch_vcpu_ioctl_run():
> 
>   ...
>   } else if (vcpu->arch.hcall_needed) {
>     int i
> 
>     kvmppc_set_gpr(vcpu, 3, run->papr_hcall.ret);
>     for (i = 0; i < 9; ++i)
>            kvmppc_set_gpr(vcpu, 4 + i, run->papr_hcall.args[i]);
>     vcpu->arch.hcall_needed = 0;
> 
> since vcpu->arch.hcall_needed == 1 indicates that userspace should
> have handled the hcall and stored the return value in
> run->papr_hcall.ret. Since that's not the case here, we can get an
> unexpected value in VCPU r3, which can result in
> kvmhv_p9_guest_entry() reporting an unexpected trap value when it
> returns from H_ENTER_NESTED, causing the following register dump to
> console via subsequent call to kvmppc_handle_exit_hv() in L1:
> 
>   [  350.612854] vcpu 00000000f9564cf8 (0):
>   [  350.612915] pc  = c00000000013eb98  msr = 8000000000009033  trap
> = 1
>   [  350.613020] r 0 = c0000000004b9044  r16 = 0000000000000000
>   [  350.613075] r 1 = c00000007cffba30  r17 = 0000000000000000
>   [  350.613120] r 2 = c00000000178c100  r18 = 00007fffc24f3b50
>   [  350.613166] r 3 = c00000007ef52480  r19 = 00007fffc24fff58
>   [  350.613212] r 4 = 0000000000000000  r20 = 00000a1e96ece9d0
>   [  350.613253] r 5 = 70616d00746f6f72  r21 = 00000a1ea117c9b0
>   [  350.613295] r 6 = 0000000000000020  r22 = 00000a1ea1184360
>   [  350.613338] r 7 = c0000000783be440  r23 = 0000000000000003
>   [  350.613380] r 8 = fffffffffffffffc  r24 = 00000a1e96e9e124
>   [  350.613423] r 9 = c00000007ef52490  r25 = 00000000000007ff
>   [  350.613469] r10 = 0000000000000004  r26 = c00000007eb2f7a0
>   [  350.613513] r11 = b0616d0009eccdb2  r27 = c00000007cffbb10
>   [  350.613556] r12 = c0000000004b9000  r28 = c00000007d83a2c0
>   [  350.613597] r13 = c000000001b00000  r29 = c0000000783cdf68
>   [  350.613639] r14 = 0000000000000000  r30 = 0000000000000000
>   [  350.613681] r15 = 0000000000000000  r31 = c00000007cffbbf0
>   [  350.613723] ctr = c0000000004b9000  lr  = c0000000004b9044
>   [  350.613765] srr0 = 0000772f954dd48c srr1 = 800000000280f033
>   [  350.613808] sprg0 = 0000000000000000 sprg1 = c000000001b00000
>   [  350.613859] sprg2 = 0000772f9565a280 sprg3 = 0000000000000000
>   [  350.613911] cr = 88002848  xer = 0000000020040000  dsisr =
> 42000000
>   [  350.613962] dar = 0000772f95390000
>   [  350.614031] fault dar = c000000244b278c0 dsisr = 00000000
>   [  350.614073] SLB (0 entries):
>   [  350.614157] lpcr = 0040000003d40413 sdr1 = 0000000000000000
> last_inst = ffffffff
>   [  350.614252] trap=0x1 | pc=0xc00000000013eb98 |
> msr=0x8000000000009033
> 
> followed by L1's QEMU reporting the following before stopping
> execution
> of the nested guest:
> 
>   KVM: unknown exit, hardware reason 1
>   NIP c00000000013eb98   LR c0000000004b9044 CTR c0000000004b9000 XER
> 0000000020040000 CPU#0
>   MSR 8000000000009033 HID0 0000000000000000  HF 8000000000000000
> iidx 3 didx 3
>   TB 00000000 00000000 DECR 00000000
>   GPR00 c0000000004b9044 c00000007cffba30 c00000000178c100
> c00000007ef52480
>   GPR04 0000000000000000 70616d00746f6f72 0000000000000020
> c0000000783be440
>   GPR08 fffffffffffffffc c00000007ef52490 0000000000000004
> b0616d0009eccdb2
>   GPR12 c0000000004b9000 c000000001b00000 0000000000000000
> 0000000000000000
>   GPR16 0000000000000000 0000000000000000 00007fffc24f3b50
> 00007fffc24fff58
>   GPR20 00000a1e96ece9d0 00000a1ea117c9b0 00000a1ea1184360
> 0000000000000003
>   GPR24 00000a1e96e9e124 00000000000007ff c00000007eb2f7a0
> c00000007cffbb10
>   GPR28 c00000007d83a2c0 c0000000783cdf68 0000000000000000
> c00000007cffbbf0
>   CR 88002848  [ L  L  -  -  E  L  G  L  ]             RES
> ffffffffffffffff
>    SRR0 0000772f954dd48c  SRR1 800000000280f033    PVR
> 00000000004e1202 VRSAVE 0000000000000000
>   SPRG0 0000000000000000 SPRG1 c000000001b00000  SPRG2
> 0000772f9565a280  SPRG3 0000000000000000
>   SPRG4 0000000000000000 SPRG5 0000000000000000  SPRG6
> 0000000000000000  SPRG7 0000000000000000
>   HSRR0 0000000000000000 HSRR1 0000000000000000
>    CFAR 0000000000000000
>    LPCR 0000000003d40413
>    PTCR 0000000000000000   DAR 0000772f95390000  DSISR
> 0000000042000000
> 
> Fix this by setting vcpu->arch.hcall_needed = 0 to indicate
> completion
> of H_ENTER_NESTED before we exit to L0 userspace.

Nice Catch :)

Reviewed-by: Suraj Jitindar Singh <[email-masked]>

> 
> Cc: [email-masked]
> Cc: David Gibson <[email-masked]>
> Cc: Paul Mackerras <[email-masked]>
> Cc: Suraj Jitindar Singh <[email-masked]>
> Signed-off-by: Michael Roth <[email-masked]>
> ---
>  arch/powerpc/kvm/book3s_hv.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c
> b/arch/powerpc/kvm/book3s_hv.c
> index d65b961661fb..a56f8413758a 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -983,6 +983,7 @@ int kvmppc_pseries_do_hcall(struct kvm_vcpu
> *vcpu)
>  		ret = kvmhv_enter_nested_guest(vcpu);
>  		if (ret == H_INTERRUPT) {
>  			kvmppc_set_gpr(vcpu, 3, 0);
> +			vcpu->arch.hcall_needed = 0;
>  			return -EINTR;
>  		}
>  		break;