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    SubjectRe: [RFC PATCH 00/26] Runtime paravirt patching
    On Wed, Apr 08, 2020 at 03:33:52PM +0200, Jürgen Groß wrote:
    > On 08.04.20 14:08, Peter Zijlstra wrote:
    > > On Tue, Apr 07, 2020 at 10:02:57PM -0700, Ankur Arora wrote:
    > > > Mechanism: the patching itself is done using stop_machine(). That is
    > > > not ideal -- text_poke_stop_machine() was replaced with INT3+emulation
    > > > via text_poke_bp(), but I'm using this to address two issues:
    > > > 1) emulation in text_poke() can only easily handle a small set
    > > > of instructions and this is problematic for inlined pv-ops (and see
    > > > a possible alternatives use-case below.)
    > > > 2) paravirt patching might have inter-dependendent ops (ex.
    > > > lock.queued_lock_slowpath, lock.queued_lock_unlock are paired and
    > > > need to be updated atomically.)
    > >
    > > And then you hope that the spinlock state transfers.. That is that both
    > > implementations agree what an unlocked spinlock looks like.
    > >
    > > Suppose the native one was a ticket spinlock, where unlocked means 'head
    > > == tail' while the paravirt one is a test-and-set spinlock, where
    > > unlocked means 'val == 0'.
    > >
    > > That just happens to not be the case now, but it was for a fair while.
    >
    > Sure? This would mean that before spinlock-pvops are being set no lock
    > is allowed to be used in the kernel, because this would block the boot
    > time transition of the lock variant to use.

    Hurm.. true. I suppose I completely forgot how paravirt spinlocks looked
    before it got rewritten.

    > Another problem I'm seeing is that runtime pvops patching would rely on
    > the fact that stop_machine() isn't guarded by a spinlock.

    It can't be, stop_machine() relies on scheduling. But yes, that another
    variation of 'stuff uses spinlocks'.

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