RE: [PATCH 07/31] nVMX: Introduce vmcs02: VMCS used to run L2

From: "Tian, Kevin" <kevin.tian@intel.com>
To: Nadav Har'El <nyh@math.technion.ac.il>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"gleb@redhat.com" <gleb@redhat.com>,
	"avi@redhat.com" <avi@redhat.com>
Subject: RE: [PATCH 07/31] nVMX: Introduce vmcs02: VMCS used to run L2
Date: Tue, 24 May 2011 08:54:10 +0800	[thread overview]
Message-ID: <625BA99ED14B2D499DC4E29D8138F1505C9BEF06CC@shsmsx502.ccr.corp.intel.com> (raw)
In-Reply-To: <20110522072231.GA186@fermat.math.technion.ac.il>

> From: Nadav Har'El [mailto:nyh@math.technion.ac.il]
> Sent: Sunday, May 22, 2011 3:23 PM
> 
> Hi,
> 
> On Sun, May 22, 2011, Tian, Kevin wrote about "RE: [PATCH 07/31] nVMX:
> Introduce vmcs02: VMCS used to run L2":
> > Here the vmcs02 being overridden may have been run on another processor
> before
> > but is not vmclear-ed yet. When you resume this vmcs02 with new content on
> a
> > separate processor, the risk of corruption exists.
> 
> I still believe that my current code is correct (in this area). I'll try to
> explain it here and would be grateful if you could point to me the error (if
> there is one) in my logic:
> 
> Nested_vmx_run() is our function which is switches from running L1 to L2
> (patch 18).
> 
> This function starts by calling nested_get_current_vmcs02(), which gets us
> *some* vmcs to use for vmcs02. This may be a fresh new VMCS, or a
> "recycled"
> VMCS, some VMCS we've previously used to run some, potentially different L2
> guest on some, potentially different, CPU.
> nested_get_current_vmcs02() returns a "saved_vmcs" structure, which
> not only contains a VMCS, but also remembers on which (if any) cpu it is
> currently loaded (and whether it was VMLAUNCHed once on that cpu).
> 
> The next thing that Nested_vmx_run() now does is to set up in the vcpu object
> the vmcs, cpu and launched fields according to what was returned above.
> 
> Now it calls vmx_vcpu_load(). This standard KVM function checks if we're now
> running on a different CPU from the vcpu->cpu, and if it a different one, is
> uses vcpu_clear() to VMCLEAR the vmcs on the CPU where it was last loaded
> (using an IPI). Only after it vmclears the VMCS on the old CPU, it can finally
> load the VMCS on the new CPU.
> 
> Only now Nested_vmx_run() can call prepare_vmcs02, which starts
> VMWRITEing
> to this VMCS, and finally returns.
> 

yes, you're correct. Previously I just looked around 07/31 and raised above concern.
Along with nested_vmx_run you explained above, this part is clear to me now. :-)

> P.S. Seeing that you're from Intel, maybe you can help me with a pointer:
> I found what appears to be a small error in the SDM - who can I report it to?
> 

Let me ask for you.

Thanks
Kevin