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

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Message-ID: <53DA7B28.701@redhat.com>
Date:	Thu, 31 Jul 2014 19:21:44 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Peter Maydell <peter.maydell@...aro.org>
CC:	Alex Bennée <alex.bennee@...aro.org>,
	"kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
	arm-mail-list <linux-arm-kernel@...ts.infradead.org>,
	kvm-devel <kvm@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	open list <linux-kernel@...r.kernel.org>,
	Gleb Natapov <gleb@...nel.org>
Subject: Re: [PATCH] arm64: KVM: export current vcpu->pause state via pseudo
 regs

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.

>> Also, how is KVM/ARM
>> representing (and passing to QEMU) the halted state of the
>> VCPU?
> 
> We don't. In ARM the equivalent of x86 HLT (which is
> WFI, wait-for-interrupt) is allowed to resume at any time.
> So we don't need to care about saving and restoring
> whether we were sat in a WFI at point of migration.

What does ARM do if you have a WFI while interrupts are disabled?  On
x86 after "cli;hlt" only an NMI will wake you up.  With spurious
wakeups, it's pretty much guaranteed that you will break such "cli;hlt"
sequences.

Paolo
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