Re: [Xen-devel] [PATCH v3] x86/vlapic: Don't reset APIC ID when handling INIT signal

[Julien Grall <julien.grall@xxxxxxx>]

On 20/04/17 10:25, Chao Gao wrote:
> On Thu, Apr 20, 2017 at 04:39:06PM +0100, Julien Grall wrote:
> > Hi,
> >
> > On 20/04/17 14:34, Jan Beulich wrote:
> > > > > > On 19.04.17 at 22:22, <chao.gao@xxxxxxxxx> wrote:
> > > > According to SDM "ADVANCED PROGRAMMABLE INTERRUPT CONTROLLER (APIC) ->
> > > > "EXTENDED XAPIC (X2APIC)" -> "x2APIC State Transitions", the APIC mode
> > > > and APIC ID are preserved when handling INIT signal and a reset places
> > > > APIC to xAPIC mode and APIC base address to 0xFEE00000h (this part
> > > > is in "Local APIC" -> "Local APIC Status and Location"). So there are
> > > > two problems in current code:
> > > > 1. Using reset logic (aka vlapic_reset) to handle INIT signal.
> > > > 2. Forgetting resetting APIC mode and base address in vlapic_reset()
> > > >
> > > > This patch introduces a new function vlapic_do_init() and replaces the
> > > > wrongly used vlapic_reset(). Also reset APIC mode and APIC base address
> > > > in vlapic_reset().
> > > >
> > > > Note that: LDR is read only in x2APIC mode. Resetting it to zero in x2APIC
> > > > mode is unreasonable. This patch also doesn't reset LDR when handling INIT
> > > > signal in x2APIC mode.
> > > >
> > > > Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> > > Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> > >
> > > > I regard this patch as a bug fix. But I haven't seen issues caused by
> > > > this bug and am not sure of the existance of such issues. Anyhow Cc
> > > > Julien and leave the decision to you (Julien and Jan).
> > > Julien,
> > >
> > > I'm slightly in favor of taking it now, but I won't object if you decide
> > > otherwise.
> > Chao, can you assess the benefits/risks of having this patch in Xen 4.9?
> I think the risk is low for only fixing SPEC mismatched behavior and
> benefit is also small as cases (changing APIC ID or Performance INIT
> operation) trigger this bug are rare.
Release-acked-by: Julien Grall <julien.grall@xxxxxxx>

> Thanks
> Chao
--
Julien Grall

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