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Thu, 11 Nov 2021 17:48:21 +0000 (GMT) Message-ID: Subject: Re: [RFC PATCH v3 2/2] KVM: s390: Extend the USER_SIGP capability From: Eric Farman To: Janosch Frank , David Hildenbrand , Christian Borntraeger , Claudio Imbrenda , Thomas Huth Cc: Heiko Carstens , Vasily Gorbik , kvm@vger.kernel.org, linux-s390@vger.kernel.org Date: Thu, 11 Nov 2021 12:48:20 -0500 In-Reply-To: <32836eb5-532f-962d-161a-faa2213a0691@linux.ibm.com> References: <20211110203322.1374925-1-farman@linux.ibm.com> <20211110203322.1374925-3-farman@linux.ibm.com> <32836eb5-532f-962d-161a-faa2213a0691@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-ORIG-GUID: tojX6H5ZY0eJh1bYwWknuJ_ufvmnymwW X-Proofpoint-GUID: XUWrA0VHIYcpaAYBd9SW27_jcegqqxGQ 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_06,2021-11-11_01,2020-04-07_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 spamscore=0 bulkscore=0 lowpriorityscore=0 adultscore=0 suspectscore=0 mlxscore=0 mlxlogscore=999 impostorscore=0 priorityscore=1501 clxscore=1015 phishscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2110150000 definitions=main-2111110093 Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Thu, 2021-11-11 at 17:13 +0100, Janosch Frank wrote: > On 11/11/21 16:03, Eric Farman wrote: > > 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. > > > > Looking at the API I'd like to avoid having two IOCTLs Since the order is a single byte, we could have the payload of an ioctl say "0-255 is an order that we're busy processing, anything higher than that resets the busy" or something. That would remove the need for a second IOCTL. > and I'd love to > see some way to extend this without the need for a whole new IOCTL. > Do you mean zero IOCTLs? Because... I think the only way we can do that is to get rid of USER_SIGP altogether, and handle everything in-kernel. Some weeks ago I played with QEMU not enabling USER_SIGP, but I can't say I've tried it since we went down this "mark a vcpu busy" path. If I do that busy/not-busy tagging in the kernel for !USER_SIGP, that might not be a bad thing anyway. But I don't know how we get the behavior straightened out for USER_SIGP without some type of handshake. > > > > > Not that we would need the value > > > right now, but who knows for what we might reuse that interface > > > in > > > the > > > future. > > > > > > Thanks! > > >