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Thu, 11 Nov 2021 15:03:13 +0000 (GMT) Message-ID: Subject: Re: [RFC PATCH v3 2/2] KVM: s390: Extend the USER_SIGP capability From: Eric Farman To: David Hildenbrand , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Thomas Huth Cc: Heiko Carstens , Vasily Gorbik , kvm@vger.kernel.org, linux-s390@vger.kernel.org Date: Thu, 11 Nov 2021 10:03:12 -0500 In-Reply-To: References: <20211110203322.1374925-1-farman@linux.ibm.com> <20211110203322.1374925-3-farman@linux.ibm.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.28.5 (3.28.5-16.el8) Mime-Version: 1.0 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: 4INnjYFx4JwrkGXBzSCK_OPMpkQx4mQo X-Proofpoint-ORIG-GUID: 4QZCR_cBqu4o_3KKnkkz_KNXRa7iH5Rz X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.790,Hydra:6.0.425,FMLib:17.0.607.475 definitions=2021-11-11_04,2021-11-11_01,2020-04-07_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 bulkscore=0 phishscore=0 spamscore=0 adultscore=0 clxscore=1011 lowpriorityscore=0 mlxlogscore=999 priorityscore=1501 mlxscore=0 malwarescore=0 impostorscore=0 suspectscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2110150000 definitions=main-2111110084 Precedence: bulk List-ID: X-Mailing-List: linux-s390@vger.kernel.org On Thu, 2021-11-11 at 10:15 +0100, David Hildenbrand wrote: > On 10.11.21 21:33, Eric Farman wrote: > > With commit 2444b352c3ac ("KVM: s390: forward most SIGP orders to > > user > > space") we have a capability that allows the "fast" SIGP orders (as > > defined by the Programming Notes for the SIGNAL PROCESSOR > > instruction in > > the Principles of Operation) to be handled in-kernel, while all > > others are > > sent to userspace for processing. > > > > This works fine but it creates a situation when, for example, a > > SIGP SENSE > > might return CC1 (STATUS STORED, and status bits indicating the > > vcpu is > > stopped), when in actuality userspace is still processing a SIGP > > STOP AND > > STORE STATUS order, and the vcpu is not yet actually stopped. Thus, > > the > > SIGP SENSE should actually be returning CC2 (busy) instead of CC1. > > > > To fix this, add another CPU capability, dependent on the USER_SIGP > > one, > > and two associated IOCTLs. One IOCTL will be used by userspace to > > mark a > > vcpu "busy" processing a SIGP order, and cause concurrent orders > > handled > > in-kernel to be returned with CC2 (busy). Another IOCTL will be > > used by > > userspace to mark the SIGP "finished", and the vcpu free to process > > additional orders. > > > > This looks much cleaner to me, thanks! > > [...] > > > diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h > > index c07a050d757d..54371cede485 100644 > > --- a/arch/s390/kvm/kvm-s390.h > > +++ b/arch/s390/kvm/kvm-s390.h > > @@ -82,6 +82,22 @@ static inline int is_vcpu_idle(struct kvm_vcpu > > *vcpu) > > return test_bit(vcpu->vcpu_idx, vcpu->kvm->arch.idle_mask); > > } > > > > +static inline bool kvm_s390_vcpu_is_sigp_busy(struct kvm_vcpu > > *vcpu) > > +{ > > + return (atomic_read(&vcpu->arch.sigp_busy) == 1); > > You can drop () > > > +} > > + > > +static inline bool kvm_s390_vcpu_set_sigp_busy(struct kvm_vcpu > > *vcpu) > > +{ > > + /* Return zero for success, or -EBUSY if another vcpu won */ > > + return (atomic_cmpxchg(&vcpu->arch.sigp_busy, 0, 1) == 0) ? 0 : > > -EBUSY; > > You can drop () as well. > > We might not need the -EBUSY semantics after all. User space can just > track if it was set, because it's in charge of setting it. Hrm, I added this to distinguish a newer kernel with an older QEMU, but of course an older QEMU won't know the difference either. I'll doublecheck that this is works fine in the different permutations. > > > +} > > + > > +static inline void kvm_s390_vcpu_clear_sigp_busy(struct kvm_vcpu > > *vcpu) > > +{ > > + atomic_set(&vcpu->arch.sigp_busy, 0); > > +} > > + > > static inline int kvm_is_ucontrol(struct kvm *kvm) > > { > > #ifdef CONFIG_KVM_S390_UCONTROL > > diff --git a/arch/s390/kvm/sigp.c b/arch/s390/kvm/sigp.c > > index 5ad3fb4619f1..a37496ea6dfa 100644 > > --- a/arch/s390/kvm/sigp.c > > +++ b/arch/s390/kvm/sigp.c > > @@ -276,6 +276,10 @@ static int handle_sigp_dst(struct kvm_vcpu > > *vcpu, u8 order_code, > > if (!dst_vcpu) > > return SIGP_CC_NOT_OPERATIONAL; > > > > + if (kvm_s390_vcpu_is_sigp_busy(dst_vcpu)) { > > + return SIGP_CC_BUSY; > > + } > > You can drop {} Arg, I had some debug in there which needed the braces, and of course it's unnecessary now. Thanks. > > > + > > switch (order_code) { > > case SIGP_SENSE: > > vcpu->stat.instruction_sigp_sense++; > > @@ -411,6 +415,12 @@ int kvm_s390_handle_sigp(struct kvm_vcpu > > *vcpu) > > if (handle_sigp_order_in_user_space(vcpu, order_code, > > cpu_addr)) > > return -EOPNOTSUPP; > > > > + /* Check the current vcpu, if it was a target from another vcpu > > */ > > + if (kvm_s390_vcpu_is_sigp_busy(vcpu)) { > > + kvm_s390_set_psw_cc(vcpu, SIGP_CC_BUSY); > > + return 0; > > + } > > I don't think we need this. I think the above (checking the target of > a > SIGP order) is sufficient. Or which situation do you have in mind? > Hrm... I think you're right. I was thinking of this: VCPU 1 - SIGP STOP CPU 2 VCPU 2 - SIGP SENSE CPU 1 But of course either CPU2 is going to be marked "busy" first, and the sense doesn't get processed until it's reset, or the sense arrives first, and the busy/notbusy doesn't matter. Let me doublecheck my tests for the non-RFC version. > > > I do wonder if we want to make this a kvm_arch_vcpu_ioctl() instead, In one of my original attempts between v1 and v2, I had put this there. This reliably deadlocks my guest, because the caller (kvm_vcpu_ioctl()) tries to acquire vcpu->mutex, and racing SIGPs (via KVM_RUN) might already be holding it. Thus, it's an async ioctl. I could fold it into the existing interrupt ioctl, but as those are architected structs it seems more natural do it this way. Or I have mis-understood something along the way? > essentially just providing a KVM_S390_SET_SIGP_BUSY *and* providing > the > order. "order == 0" sets it to !busy. I'd tried this too, since it provided some nice debug-ability. Unfortunately, I have a testcase (which I'll eventually get folded into kvm-unit-tests :)) that picks a random order between 0-255, knowing that there's only a couple handfuls of valid orders, to check the response. Zero is valid architecturally (POPS figure 4-29), even if it's unassigned. The likelihood of it becoming assigned is probably quite low, but I'm not sure that I like special-casing an order of zero in this way. > Not that we would need the value > right now, but who knows for what we might reuse that interface in > the > future. > > Thanks! >