>>> On 17.03.14 at 17:55, "H. Peter Anvin" <h...@zytor.com> wrote: > On 03/17/2014 05:19 AM, George Dunlap wrote: >> On Mon, Mar 17, 2014 at 3:33 AM, H. Peter Anvin <h...@zytor.com> wrote: >>> No, the right thing is to unf*ck the Xen braindamage and use eagerfpu as a > workaround for the legacy hypervisor versions. >> >> The interface wasn't an accident. In the most common case you'll want >> to clear the bit anyway. In PV mode clearing it would require an extra >> trip up into the hypervisor. So this saves one trip up into the >> hypervisor on every context switch which involves an FPU, at the >> expense of not being able to context-switch away when handling the >> trap. > > The interface was a complete faceplant, because it caused failures. > You're not infinitely unconstrained since you want to play in the same > sandbox as the native architecture, and if you want to have a hope of > avoiding these kinds of failures you really need to avoid making random > "improvements", certainly not without an explicit guest opt-in (the same > we do for the native CPU architecture when adding new features.) > > So if this interface wasn't an accident it was active negligence and > incompetence.
I don't think so - while it (as we now see) disallows certain things inside the guest, back at the time when this was designed there was no sign of any sort of allocation/scheduling being done inside the #NM handler. And furthermore, a PV specification is by its nature allowed to define deviations from real hardware behavior, or else it wouldn't be needed in the first place. Jan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
