On 02/28/2012 09:26 PM, Linus Torvalds wrote: > On Tue, Feb 28, 2012 at 11:06 AM, Avi Kivity <avi@xxxxxxxxxx> wrote: > > > > No, the scheduler saves the state into task_struct. I need it saved > > into the vcpu structure. We have two fpu states, the user state, and > > the guest state. APIs that take a task_struct as a parameter, or > > reference current implicitly, aren't going to work. > > As far as I can tell, you should do the saving into the vcpu structure > when you actually switch the thing around. > > In fact, you can do it these days by just playing around with the > "tsk->thread.fpu.state" pointer, I guess. Good idea. I can't say I like poking into struct fpu's internals, but we can treat it as an opaque structure and copy it around. We can also do this in kernel_fpu_begin(), and allow it to be preemptible. > But it all boils down to the fact that your code is not just ugly, > it's *buggy*. If you play around with setting TS, you *will* be hit by > interrupts etc that will start to use the FP code that you "don't > use". > > And there is no excuse for you touching the host TS. The kernel does > that for you, and does it better. And caches the end result in > TS_USEDFPU (old) or in some variable that you shouldn't look at but > can access with the user_has_fpu() helpers. Again, I can't avoid touching it. I can try to get the hardware to always preserve its value, but that comes with a cost. btw, I think the current code is safe wrt kvm. If the guest fpu has been loaded, then we know that that TS_USEDFPU is set, since we will have saved the user fpu earlier. Yes it's "accidental" and needs to be improved, but I don't think it's a data corruptor waiting to happen. -- error compiling committee.c: too many arguments to function -- 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
