linux-kernel - Re: [PATCH] arm64: KVM: export current vcpu->pause state via pseudo regs

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Date:	Mon, 4 Aug 2014 14:16:55 +0200
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Alex Bennee <alex@...nee.com>
Cc:	Peter Maydell <peter.maydell@...aro.org>,
	kvm-devel <kvm@...r.kernel.org>, Gleb Natapov <gleb@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	open list <linux-kernel@...r.kernel.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH] arm64: KVM: export current vcpu->pause state via pseudo
 regs

On Thu, Jul 31, 2014 at 07:21:44PM +0200, Paolo Bonzini wrote:
> Il 31/07/2014 19:04, Peter Maydell ha scritto:
> > On 31 July 2014 17:57, Paolo Bonzini <pbonzini@...hat.com> wrote:
> >> Il 09/07/2014 15:55, Alex Bennée ha scritto:
> >>> To cleanly restore an SMP VM we need to ensure that the current pause
> >>> state of each vcpu is correctly recorded. Things could get confused if
> >>> the CPU starts running after migration restore completes when it was
> >>> paused before it state was captured.
> >>>
> >>> I've done this by exposing a register (currently only 1 bit used) via
> >>> the GET/SET_ONE_REG logic to pass the state between KVM and the VM
> >>> controller (e.g. QEMU).
> >>>
> >>> Signed-off-by: Alex Bennée <alex.bennee@...aro.org>
> >>> ---
> >>>  arch/arm64/include/uapi/asm/kvm.h |  8 +++++
> >>>  arch/arm64/kvm/guest.c            | 61 ++++++++++++++++++++++++++++++++++++++-
> >>>  2 files changed, 68 insertions(+), 1 deletion(-)
> >>
> >> Since it's a pseudo register anyway, would it make sense to use the
> >> existing KVM_GET/SET_MP_STATE ioctl interface?
> > 
> > That appears to be an x86-specific thing relating to
> > IRQ chips.
> 
> No, it's not.  It's just the state of the CPU, s390 will be using it too.
> 
> On x86 the states are uninitialized (UNINITIALIZED), stopped
> (INIT_RECEIVED), running (RUNNABLE), halted (HALTED).  CPU 0 starts in
> RUNNABLE state, other CPUs start in UNINITIALIZED state.  There are
> x86-specific cases (uninitialized) and x86-isms (the INIT_RECEIVED
> name), but the idea is widely applicable.
> 
Alex, I think it makes perfect sense to use GET/SET_MP_STATE, will you
revise the patch?

(Don't forget to update the documentation to reflect it is now supported
on ARM, and which states are used to represent what there.)

Thanks,
-Christoffer
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