From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:47120) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fQxc8-0004p9-Gg for qemu-devel@nongnu.org; Thu, 07 Jun 2018 12:18:19 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fQxc5-0002xV-5j for qemu-devel@nongnu.org; Thu, 07 Jun 2018 12:18:16 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:59180 helo=mx0a-001b2d01.pphosted.com) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fQxc4-0002vc-VL for qemu-devel@nongnu.org; Thu, 07 Jun 2018 12:18:13 -0400 Received: from pps.filterd (m0098421.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w57G9j7Y053331 for ; Thu, 7 Jun 2018 12:18:04 -0400 Received: from e06smtp05.uk.ibm.com (e06smtp05.uk.ibm.com [195.75.94.101]) by mx0a-001b2d01.pphosted.com with ESMTP id 2jf4uyjgm0-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Thu, 07 Jun 2018 12:18:04 -0400 Received: from localhost by e06smtp05.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 7 Jun 2018 17:18:02 +0100 References: <20180509154822.23510-1-cohuck@redhat.com> <20180509154822.23510-3-cohuck@redhat.com> <20180515181006.0cb1dfc2.cohuck@redhat.com> <20180522145208.310143ea.cohuck@redhat.com> <4e4001cc-540e-0f2b-bbd1-1f82ca594bb3@linux.ibm.com> <20180605151449.22aafbfc.cohuck@redhat.com> <20180606142131.74ea2eb7.cohuck@redhat.com> <5b77ec9c-41b8-2e32-ce79-d9005b93fdd0@linux.ibm.com> <20180607115442.6a779ed9.cohuck@redhat.com> From: Halil Pasic Date: Thu, 7 Jun 2018 18:17:57 +0200 MIME-Version: 1.0 In-Reply-To: <20180607115442.6a779ed9.cohuck@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Message-Id: <10c8a0ac-fe61-d7c7-c7bb-0fffc6909cb3@linux.ibm.com> Subject: Re: [Qemu-devel] [qemu-s390x] [PATCH RFC 2/2] vfio-ccw: support for halt/clear subchannel List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Cornelia Huck , Pierre Morel Cc: linux-s390@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, qemu-devel@nongnu.org, qemu-s390x@nongnu.org, Dong Jia Shi On 06/07/2018 11:54 AM, Cornelia Huck wrote: > Hm, I think we need to be more precise as to what scsw we're talking > about. Bad ascii art time: > > -------------- > | scsw(g) | ssch > -------------- | > | guest > -------------------------------------------------------------- > | qemu > -------------- v > | scsw(q) | emulate > -------------- | > | > -------------- v > | scsw(r) | pwrite() > -------------- | > | > -------------------------------------------------------------- > | vfio > v > ssch > | > -------------------------------------------------------------- > | hardware > -------------- v > | scsw(h) | actually do something > -------------- > > The guest issues a ssch (which gets intercepted; it won't get control > back until ssch finishes with a cc set.) scsw(g) won't change, unless > the guest does a stsch for the subchannel on another vcpu, in which > case it will get whatever information qemu holds in scsw(q) at that > point in time. (1) I think BQL make other cpu or not other kind of the same. We will effectively start processing the stsch in QEMU after we are done with the ssch in QEMU. > > When qemu starts to emulate the guest's ssch, it will set the start > function bit in the fctl field of scsw(q). It then copies scsw(q) to > scsw(r) in the vfio region. > (2) This is architecturally wrong AFAIK. The fctl bit is supposed to be set on cc 0. But because of (1) this might not be a observable by the guest -- we can fix it up. (3)IMHO scsw(r) is not a real scsw as defined by the architecture but a strange communication structure (not) defined vfio-ccw. > The vfio code will then proceed to call ssch on the real subchannel. > This is the first time we get really asynchronous, as the ssch will > return with cc set and the start function will be performed at some > point in time. If we would do a stsch on the real subchannel, we would > see that scsw(h) now has the start function bit set. > (4) I guess only if cc 0. > Currently, we won't return back up the chain until we get an interrupt > from the hardware, at which time we update the scsw(r) from the irb. > This will propagate into the scsw(q). At the time we finish handling > the guest's ssch and return control to it, we're all done and if the > guest does a stsch to update its scsw(g), it will get the current > scsw(q) which will already contain the scsw from the interrupt's irb > (indicating that the start function is already finished). > > Now let's imagine we have a future implementation that handles actually > performing the start on the hardware asynchronously, i.e. it returns > control to the guest without the interrupt having been posted (let's > say that it is a longer-running I/O request). If the guest now did a > stsch to update scsw(g), it would get the current state of scsw(q), > which would be "start function set, but not done yet". (5) AFAIK this is how the current implementation works. We don't wait for the I/O interrupt on the host to present a cc to the guest for it's ssch. > > If the guest now does a hsch, it would trap in the same way as the ssch > before. When qemu gets control, it adds the halt bit in scsw(q) (which > is in accordance with the architecture). (7) Again it's when is fctl set according to the architecture... > My proposal is to do the same > copying to scsw(r) again, which would mean we get a request with both > the halt and the start bit set. (8) IMHO when receiving the 'request' we are and should be in instruction context -- opposed to basic io function context. So we should not set fctl before we know what will our guest cc be. But since scsw(r) is not a real scsw it is just strange. > The vfio code now needs to do a hsch > (instead of a ssch). The real channel subsystem should figure this out, > as we can't reliably check whether the start function has concluded > already (there's always a race window). > (9) Yes we can't tell for sure if the start function is still being performed by the stuff below. Regards, Halil > For csch, things are a bit different (which the code posted here did > not take into account). The qemu emulation of csch needs to clear any > start/halt bits in scsw(q) when setting the clear bit there, and > therefore scsw(r) will only have the clear bit set in that case. We > still should do an unconditional csch for the same reasons as above; > the hardware will do the same things (clearing start/halt, setting > clear) in the scsw(h). > > Congratulations, you've reached the end:) I hope that was helpful and > not too confusing. >