Re: [RFC/PATCH 1/3] powerpc/kvm: Handle H_FAC_UNAVAIL when guest executes stop.

[Nicholas Piggin]

Gautham R. Shenoy's on March 31, 2020 10:10 pm:
> From: "Gautham R. Shenoy" 
> 
> If a guest executes a stop instruction when the hypervisor has set the
> PSSCR[ESL|EC] bits, the processor will throw an Hypervisor Facility
> Unavailable exception. Currently when we receive this exception, we
> only check if the exeception is generated due to a doorbell
> instruction, in which case we emulate it. For all other cases,
> including the case when the guest executes a stop-instruction, the
> hypervisor sends a PROGILL to the guest program, which results in a
> guest crash.
> 
> This patch adds code to handle the case when the hypervisor receives a
> H_FAC_UNAVAIL exception due to guest executing the stop
> instruction. The hypervisor increments the pc to the next instruction
> and resumes the guest as expected by the semantics of the
> PSSCR[ESL|EC] = 0 stop instruction.

This seems reasonable, I don't think we need to crash the guest here.

Thanks,
Nick

[RFC/PATCH 1/3] powerpc/kvm: Handle H_FAC_UNAVAIL when guest executes stop.

[Gautham R. Shenoy]

From: "Gautham R. Shenoy" 

If a guest executes a stop instruction when the hypervisor has set the
PSSCR[ESL|EC] bits, the processor will throw an Hypervisor Facility
Unavailable exception. Currently when we receive this exception, we
only check if the exeception is generated due to a doorbell
instruction, in which case we emulate it. For all other cases,
including the case when the guest executes a stop-instruction, the
hypervisor sends a PROGILL to the guest program, which results in a
guest crash.

This patch adds code to handle the case when the hypervisor receives a
H_FAC_UNAVAIL exception due to guest executing the stop
instruction. The hypervisor increments the pc to the next instruction
and resumes the guest as expected by the semantics of the
PSSCR[ESL|EC] = 0 stop instruction.

Signed-off-by: Gautham R. Shenoy 
---
 arch/powerpc/include/asm/reg.h | 1 +
 arch/powerpc/kvm/book3s_hv.c   | 6 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/reg.h b/arch/powerpc/include/asm/reg.h
index da5cab0..2568c18 100644
--- a/arch/powerpc/include/asm/reg.h
+++ b/arch/powerpc/include/asm/reg.h
@@ -399,6 +399,7 @@
 /* HFSCR and FSCR bit numbers are the same */
 #define FSCR_SCV_LG12  /* Enable System Call Vectored */
 #define FSCR_MSGP_LG   10  /* Enable MSGP */
+#define FSCR_STOP_LG9   /* Enable stop states */
 #define FSCR_TAR_LG8   /* Enable Target Address Register */
 #define FSCR_EBB_LG7   /* Enable Event Based Branching */
 #define FSCR_TM_LG 5   /* Enable Transactional Memory */
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 33be4d9..cdb7224 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -1419,7 +1419,11 @@ static int kvmppc_handle_exit_hv(struct kvm_run *run, 
struct kvm_vcpu *vcpu,
if (((vcpu->arch.hfscr >> 56) == FSCR_MSGP_LG) &&
cpu_has_feature(CPU_FTR_ARCH_300))
r = kvmppc_emulate_doorbell_instr(vcpu);
-   if (r == EMULATE_FAIL) {
+   else if (((vcpu->arch.hfscr >> 56) == FSCR_STOP_LG) &&
+   cpu_has_feature(CPU_FTR_ARCH_300)) {
+   kvmppc_set_pc(vcpu, kvmppc_get_pc(vcpu) + 4);
+   r = RESUME_GUEST;
+   } else if (r == EMULATE_FAIL) {
kvmppc_core_queue_program(vcpu, SRR1_PROGILL);
r = RESUME_GUEST;
}
-- 
1.9.4