Re: [PATCH v11 2/3] x86, apicv: add virtual x2apic support

From: Marcelo Tosatti <mtosatti@redhat.com>
To: Gleb Natapov <gleb@redhat.com>
Cc: Yang Zhang <yang.z.zhang@intel.com>,
	kvm@vger.kernel.org, haitao.shan@intel.com,
	xiantao.zhang@intel.com, Kevin Tian <kevin.tian@intel.com>
Subject: Re: [PATCH v11 2/3] x86, apicv: add virtual x2apic support
Date: Mon, 21 Jan 2013 22:33:46 -0200	[thread overview]
Message-ID: <20130122003346.GA26201@amt.cnet> (raw)
In-Reply-To: <20130121221618.GE10985@amt.cnet>

On Mon, Jan 21, 2013 at 08:16:18PM -0200, Marcelo Tosatti wrote:
> On Mon, Jan 21, 2013 at 11:34:20PM +0200, Gleb Natapov wrote:
> > On Mon, Jan 21, 2013 at 07:21:13PM -0200, Marcelo Tosatti wrote:
> > > On Mon, Jan 21, 2013 at 10:21:14PM +0200, Gleb Natapov wrote:
> > > > > >  	}
> > > > > > +
> > > > > > +	vcpu->arch.apic_base = value;
> > > > > 
> > > > > Simpler to have
> > > > > 
> > > > > if (apic_x2apic_mode(apic)) {
> > > > > 	...
> > > > > 	kvm_x86_ops->set_virtual_x2apic_mode(vcpu, true);
> > > > > } else {
> > > > > 	kvm_x86_ops->set_virtual_x2apic_mode(vcpu, false);
> > > > > }
> > > > > 
> > > > This will not work during cpu init. That was discussed on one of
> > > > the previous iterations of the patch. When this code is called during
> > > > vcpu init vmcs is not loaded yet so set_virtual_x2apic_mode() cannot
> > > > write into it.
> > > 
> > > Are you saying that the logic to write on bit value change is due to 
> > > ordering with cpu init or that the callback is at the wrong place?
> > > 
> > The logic is because of ordering with cpu init.
> 
> OK. Still must move this conditional callback after assignment of apic_base.
> 
> > > > > Why not disable write intercept for all MSRs which represent APIC registers
> > > > > that are virtualized? Why TPR is special?
> > > > > 
> > > > This patch goes before vid is enabled. At this point only TPR is
> > > > vitalized. If APIC_WRITE exit will be generated on unhandled MSR write
> > > > then we can disable intercept for all x2apic MSRs here.
> > > 
> > > -ENOPARSE, please be more verbose. "unhandled MSR write" ?
> > By unhandled I mean unintercepted. Write to x2apic MSR will either
> > generate MSR write exit if msr bitmap says so and then x2apic MSR will
> > be handled just like today, or, if we disable intercept, it will
> > generate APIC_WRITE exit (need to consult SDM here, not sure about it).
> > One is not really preferable to the other.
> 
> It will generate APIC_WRITE, for example, if due to EOI virtualization
> exiting.

Err, no, EOI induced vmexit is due to EOI virtualization.

> The question is, why is intercept for EOI MSR address (0x80B) not being
> disabled here, while TPR is? I don't see intercept disabled by other
> patches either.

Point still valid: why intercept for EOI MSR address not being disabled?