From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([209.51.188.92]:46827) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1goQAM-00005j-Ta for qemu-devel@nongnu.org; Tue, 29 Jan 2019 04:58:53 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1goQAL-0006aY-07 for qemu-devel@nongnu.org; Tue, 29 Jan 2019 04:58:50 -0500 Date: Tue, 29 Jan 2019 10:58:40 +0100 From: Cornelia Huck Message-ID: <20190129105840.1bf46c3b.cohuck@redhat.com> In-Reply-To: <20190128203000.6dbf7ba3@oc2783563651> References: <20190121110354.2247-1-cohuck@redhat.com> <20190121110354.2247-3-cohuck@redhat.com> <2dac6201-9e71-b188-0385-d09d05071a1c@linux.ibm.com> <5627cb78-22b3-0557-7972-256bc9560d86@linux.ibm.com> <20190125112437.2c06fac6.cohuck@redhat.com> <20190125135835.2d59b511.cohuck@redhat.com> <20190125150101.3b61f0a1@oc2783563651> <20190125152154.05120461.cohuck@redhat.com> <20190125170404.28c61eab@oc2783563651> <20190128181355.2d79cae7.cohuck@redhat.com> <20190128203000.6dbf7ba3@oc2783563651> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v2 2/5] vfio-ccw: concurrent I/O handling List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Halil Pasic Cc: linux-s390@vger.kernel.org, Eric Farman , Alex Williamson , Pierre Morel , kvm@vger.kernel.org, Farhan Ali , qemu-devel@nongnu.org, qemu-s390x@nongnu.org On Mon, 28 Jan 2019 20:30:00 +0100 Halil Pasic wrote: > On Mon, 28 Jan 2019 18:13:55 +0100 > Cornelia Huck wrote: > > > On Fri, 25 Jan 2019 17:04:04 +0100 > > Halil Pasic wrote: > > > > > Do we expect userspace/QEMU to fence the bad scenarios as tries to do > > > today, or is this supposed to change to hardware should sort out > > > requests whenever possible. > > > > Does my other mail answer that? > > Sorry, I can't find the answer in your other (Date: Mon, 28 Jan 2019 > 17:59:10 +0100, Message-Id: <20190128175910.5d9677e7@oc2783563651>) mail. > AFAIU that mail talks abut the kernel and not about the userspace. > > I guess the answer is we don't expect changes to userspace, so we do > expect userspace to fence bad scenarios. Then, I really have no idea what you are aiming at with your comment :( > > > > > > The problem I see with the let the hardware sort it out is that, for > > > that to work, we need to juggle multiple translations simultaneously > > > (or am I wrong?). Doing that does not appear particularly simple to > > > me. > > > > None in the first stage, at most two in the second stage, I guess. > > > > Expected benefit of the second stage over the first stage? (I see none.) Making something possible that is allowed by the architecture. Not really important, though. > > > > Furthermore we would go through all that hassle knowingly that the > > > sole reason is working around bugs. We still expect our Linux guests > > > serializing it's ssch() stuff as it does today. Thus I would except > > > this code not getting the love nor the coverage that would guard > > > against bugs in that code. > > > > So, we should have test code for that? (Any IBM-internal channel I/O > > exercisers that may help?) > > > > None that I'm aware of. Anyone else? > > But the point I was trying to make is the following: I prefer keeping > the handling for the case "ssch()'s on top of each other" as trivial as > possible. (E.g. bail out if CP_PENDING without doing any translation.) > > > We should not rely on the guest being sane, although Linux probably is > > in that respect. > > > > I agree 100%: we should not rely on either guest or userspace emulator > being sane. But IMHO we should handle insanity with the least possible > investment. We probably disagree what the least possible investment is.