Re: [PATCH v2 3/4] VMX: Assign the right value to 'NDST' field in a concern case

From: "Wu, Feng" <feng.wu@intel.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
	"keir@xen.org" <keir@xen.org>,
	"george.dunlap@eu.citrix.com" <george.dunlap@eu.citrix.com>,
	"andrew.cooper3@citrix.com" <andrew.cooper3@citrix.com>,
	"dario.faggioli@citrix.com" <dario.faggioli@citrix.com>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>,
	"Wu, Feng" <feng.wu@intel.com>
Subject: Re: [PATCH v2 3/4] VMX: Assign the right value to 'NDST' field in a concern case
Date: Tue, 31 May 2016 10:27:34 +0000	[thread overview]
Message-ID: <E959C4978C3B6342920538CF579893F019700A43@SHSMSX103.ccr.corp.intel.com> (raw)
In-Reply-To: <57486F0402000078000EF404@prv-mh.provo.novell.com>

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@suse.com]
> Sent: Friday, May 27, 2016 10:00 PM
> To: Wu, Feng <feng.wu@intel.com>
> Cc: andrew.cooper3@citrix.com; dario.faggioli@citrix.com;
> george.dunlap@eu.citrix.com; Tian, Kevin <kevin.tian@intel.com>; xen-
> devel@lists.xen.org; konrad.wilk@oracle.com; keir@xen.org
> Subject: Re: [PATCH v2 3/4] VMX: Assign the right value to 'NDST' field in a
> concern case
> 
> >>> On 26.05.16 at 15:39, <feng.wu@intel.com> wrote:
> > Normally, in vmx_cpu_block() 'NDST' filed should have the same
> > value with 'dest' or 'MASK_INSR(dest, PI_xAPIC_NDST_MASK)' depending
> > on whether x2apic is enabled. However, in the following scenario,
> > 'NDST' has different value:
> >
> > 'vcpu_block' hook gets assigned in vmx_pi_hooks_assign(), but all
> > other three PI hooks have not been assigned or not been excuted yet.
> > And during this interval, we are running in vmx_vcpu_block(), then
> > 'NDST' may have different value.
> 
> How about simply assigning vcpu_block last? Or alternatively
> holding pi_blocking_list_lock around all four assignments? Or
> (maybe in addition to one of these) writing something sensible in
> vmx_pi_hooks_assign() before doing the hook assignments? 

The problem is vmx_vcpu_block() can still get called first, since
the other ones might not get called yet.

> Of course much depends on whether the ASSERT() you replace was
> meant just as a sanity check, or instead logic elsewhere relies on
> NDST having the right value already before getting into
> vmx_vcpu_block().

The point is we need to make sure NDST have the right value
after leaving vmx_vcpu_block().

> 
> Also, rather than saying twice that NDST has a different value in
> that case, how about stating what exact value it has then and
> why that's not what we want/need? (Same goes for the code
> comment then.)

Sure, will add this in the next version.

> 
> > This patch fix this concern case.
> 
> Here as well as in earlier patches - do you perhaps mean "corner
> case"?

Sorry for the typo.

Thanks,
Feng

> 
> Jan

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