On 21/12/2016 14:14, Eduardo Habkost wrote: > On Wed, Dec 21, 2016 at 12:21:44PM +0100, Paolo Bonzini wrote: >> >> >> On 20/12/2016 18:43, Eduardo Habkost wrote: >>> This moves the KVM and Xen files to the an accel/ subdir. >>> >>> Instead of moving the *-stubs.c file to accel/ as-is, I tried to >>> move most of the stub code to libqemustub.a. This way the obj-y >>> logic for accel/ is simpler: obj-y includes accel/ only if >>> CONFIG_SOFTMMU is set. >>> >>> The Xen stubs could be moved completely to stubs/, but some of >>> the KVM stubs depend on cpu.h. So most of the kvm-stub.c code was >>> moved to stubs/kvm.c, but some of that code was kept in >>> accel/kvm-stub.c. >> >> I think we need to decide what libqemustub is for. >> >> The original purpose was to provide different implementations of some >> functions for tools vs. emulators or (more rarely) for user-mode vs. >> system emulation. > > So, is sysemu vs user-mode a valid reason for using libqemustub? Yes, but I was thinking of a different distinction. You'd use libqemustub if user-mode emulation (or tools) only needs 2-3 functions out of a large file, while system-mode emulation needs all of it. For example, of the entire monitor API, the tools need pretty much nothing but monitor_init, monitor_get_fd, cur_mon and monitor_cur_is_qmp. Such a small extract of the API makes little sense except for "this is what is needed to compile the tools", so it's stubs/ rather than monitor-stub.c. Instead, non-KVM targets need a stub implementation of the entire API, so it's kvm-stub.c rather than stubs/kvm.c (kvm-stub.c depends on cpu.h but that's really only needed to compile it---the kvm-stub.c code actually has no dependency). There are certainly cases where libqemustub is used instead of lnot. In the specific case of sysemu vs. user-mode, stubs/cpus.c and stubs/replay-user.c should not be in libqemustub. They should be in a separate file user-exec-stub.c, which is only used if !CONFIG_SOFTMMU. > The main reason I have moved some code to sbus/kvm.c is to avoid > having to include accel/kvm-stub.c in *-user. What's wrong with ifeq ($(CONFIG_SOFTMMU),y) obj-$(CONFIG_KVM) += kvm-all.c obj-$(call lnot, $(CONFIG_KVM)) += kvm-stub.c endif similar to what is done already in Makefile.objs? > Moving xen-stub.c to libqemustub, on the other hand, is really > unnecessary. Why would it be different? >> In general, I think libqemustub should be the last resort. If possible, >> inlines in headers should be the first choice, and stubs in objs-y or >> common-objs-y (using $(call lnot) in the Makefile) should be the second. > > I understand the reasoning, but I fail to see cases when > libqemustub would be considered appropriate. Using stubs in > obj-y/common-obj-y using $(call lnot) is always possible, isn't > it? > > Hmm, maybe on cases where the decision to use the stub doesn't > depend on a single build variable (e.g. a function implemented by > a handful of targets, but not all of them). This is a good one. > Are there other examples? Does the one above (extract a small part of an API) make sense? libqemustub is a necessary evil and it's almost never necessary. It basically exists for cases where you cannot replace a source file with another wholesale. There are also some cases of premature optimization. For example reset handlers are stubbed, but: 1) system emulation implements them in vl.c which is an antipattern of its own, and 2) they are small enough that including them in user-mode emulators (together with the rest of qdev) is not a big deal. (I'm planning to remove some stubs in 2.9, so I'm taking these examples from that branch). Paolo -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
