On Fri, Feb 26, 2010 at 01:28:29PM +0100, Alexander Graf wrote: > > On 26.02.2010, at 13:25, Joerg Roedel wrote: > > > On Fri, Feb 26, 2010 at 12:28:24PM +0200, Avi Kivity wrote: > >>> +static void add_msr_offset(u32 offset) > >>> +{ > >>> + u32 old; > >>> + int i; > >>> + > >>> +again: > >>> + for (i = 0; i< MSRPM_OFFSETS; ++i) { > >>> + old = msrpm_offsets[i]; > >>> + > >>> + if (old == offset) > >>> + return; > >>> + > >>> + if (old != MSR_INVALID) > >>> + continue; > >>> + > >>> + if (cmpxchg(&msrpm_offsets[i], old, offset) != old) > >>> + goto again; > >>> + > >>> + return; > >>> + } > >>> + > >>> + /* > >>> + * If this BUG triggers the msrpm_offsets table has an overflow. Just > >>> + * increase MSRPM_OFFSETS in this case. > >>> + */ > >>> + BUG(); > >>> +} > >> > >> Why all this atomic cleverness? The possible offsets are all > >> determined statically. Even if you do them dynamically (makes sense > >> when considering pmu passthrough), it's per-vcpu and therefore > >> single threaded (just move msrpm_offsets into vcpu context). > > > > The msr_offset table is the same for all guests. It doesn't make sense > > to keep it per vcpu because it will currently look the same for all > > vcpus. For standard guests this array contains 3 entrys. It is marked > > with __read_mostly for the same reason. > > I'm still not convinced on this way of doing things. If it's static, > make it static. If it's dynamic, make it dynamic. Dynamically > generating a static list just sounds plain wrong to me.
Stop. I had a static list in the first version of the patch. This list was fine except the fact that a developer needs to remember to update this list if the list of non-intercepted msrs is expanded. The whole reason for a dynamically built list is to take the task of maintaining the list away from the developer and remove a possible source of hard to find bugs. This is what the current approach does. Joerg -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html