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Fri, 12 Nov 2021 16:09:32 +0000 (GMT) Received: from farman-thinkpad-t470p (unknown [9.163.6.3]) by b03ledav004.gho.boulder.ibm.com (Postfix) with ESMTP; Fri, 12 Nov 2021 16:09:32 +0000 (GMT) Message-ID: <42b1d53a11106f9f8e9d9b34a41b6bba738eb410.camel@linux.ibm.com> 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: Fri, 12 Nov 2021 11:09:31 -0500 In-Reply-To: <97f3e32d-e4e4-e54e-1269-1ff66828f04f@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> <97f3e32d-e4e4-e54e-1269-1ff66828f04f@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: jqRVdbgDsin7qIRzEdFPIpAXwU33uzFh X-Proofpoint-ORIG-GUID: 2R3usuklsZ8GnDB673x5-i0ksOzDia2_ 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=2021-11-12_05,2021-11-12_01,2020-04-07_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 spamscore=0 suspectscore=0 impostorscore=0 phishscore=0 mlxlogscore=999 adultscore=0 malwarescore=0 priorityscore=1501 bulkscore=0 lowpriorityscore=0 mlxscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2110150000 definitions=main-2111120092 Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Fri, 2021-11-12 at 09:49 +0100, Janosch Frank wrote: > On 11/11/21 18:48, Eric Farman wrote: > > 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. > > I'd move over to a very small struct argument with a command and a > flags > field so we can extend the IOCTL at a later time without the need to > introduce a new IOCTL. > IMHO there's no real need to make this IOCTL as small as possible > and > only handle an int as the argument with < 0 shenanigans. We should > rather focus on making this a nice and sane API if we have the option > to > do so. > So then naming this as "USER_SIGP_BUSY" is too narrow. What about just "USER_BUSY" and something like this? enum user_busy_function { SET, RESET, } enum user_busy_reason { SIGP, } struct user_busy { int function; int reason; long payload; // Optional? In our case, SIGP order } > > > > > > > > Not that we would need the value > > > > > right now, but who knows for what we might reuse that > > > > > interface > > > > > in > > > > > the > > > > > future. > > > > > > > > > > Thanks! > > > > >