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Re: [PATCH] cpu: Add starts_halted() method

From: Eduardo Habkost
Subject: Re: [PATCH] cpu: Add starts_halted() method
Date: Wed, 8 Jul 2020 12:03:16 -0400

On Wed, Jul 08, 2020 at 04:32:51PM +0100, Peter Maydell wrote:
> On Wed, 8 Jul 2020 at 16:25, Eduardo Habkost <ehabkost@redhat.com> wrote:
> > On Wed, Jul 08, 2020 at 02:14:03PM +0100, Peter Maydell wrote:
> > > The original bug as described in the commit message sounds
> > > to me like something we should look to fix in the implementation
> > > of async_run_on_cpu() -- it shouldn't cause a CPU that's halfway
> > > through reset to do a KVM_RUN or otherwise run guest code,
> > > whether that CPU is going to start powered-up or powered-down.
> >
> > What "halfway through reset" means, exactly?  Isn't halted==1
> > enough to indicate the CPU is in that state?
> I mean "while we're in the middle of the CPU method that's
> called by cpu_reset()". "halted==1" says "the CPU is halted";
> that's not the same thing. KVM_RUN happening
> as a side effect in the middle of that code is a bug
> whether the CPU happens to be intended to be put into the
> halted state or not. If the CPU is intended to be created
> not-halted then KVM_RUN can happen after cpu reset
> completes, but not before.

Wait, I thought we already had mechanisms to prevent that from
happening.  Otherwise, it would never be safe for cpu_reset() to
touch the CPU registers.


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