On Mon, 28 Jan 2019 20:30:00 +0100 Halil Pasic <pa...@linux.ibm.com> wrote:
> On Mon, 28 Jan 2019 18:13:55 +0100 > Cornelia Huck <coh...@redhat.com> wrote: > > > On Fri, 25 Jan 2019 17:04:04 +0100 > > Halil Pasic <pa...@linux.ibm.com> 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.
