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    SubjectRe: [Xen-devel] [PATCHv1] x86: don't schedule when handling #NM exception
    On 03/17/2014 05:19 AM, George Dunlap wrote:
    > On Mon, Mar 17, 2014 at 3:33 AM, H. Peter Anvin <hpa@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.
    >
    > -George
    >

    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.

    -hpa



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