[PATCH] Clear VMCS if reusing after vcpu reset

[PATCH] Clear VMCS if reusing after vcpu reset

[Tian, Kevin]

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Clear whole page when VMCS is reused due to cpu hotplug.
construct_vmcs doesn't take care of all fields and some stale
content may make hardware unhappy.

Signed-off-by Kevin Tian <kevin.tian@intel.com>

Thanks,
Kevin

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Clear whole page when VMCS is reused due to cpu hotplug.
construct_vmcs doesn't take care of all fields and some stale
content may make hardware unhappy.

Signed-off-by Kevin Tian <kevin.tian@intel.com>

diff -r d6973889288f -r ce3cbcacd650 xen/arch/x86/hvm/vmx/vmcs.c
--- a/xen/arch/x86/hvm/vmx/vmcs.c	Thu Apr 19 14:03:10 2007 -0400
+++ b/xen/arch/x86/hvm/vmx/vmcs.c	Thu Apr 19 14:13:16 2007 -0400
@@ -468,10 +468,14 @@ int vmx_create_vmcs(struct vcpu *v)
     {
         if ( (v->arch.hvm_vmx.vmcs = vmx_alloc_vmcs()) == NULL )
             return -ENOMEM;
-
-        __vmx_clear_vmcs(v);
-    }
-
+    }
+    else
+    {
+        memset(v->arch.hvm_vmx.vmcs, 0, PAGE_SIZE);
+        v->arch.hvm_vmx.vmcs->vmcs_revision_id = vmcs_revision_id;
+    }
+
+    __vmx_clear_vmcs(v);
     construct_vmcs(v);
 
     return 0;

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Re: [PATCH] Clear VMCS if reusing after vcpu reset

[Keir Fraser]

On 30/4/07 07:09, "Tian, Kevin" <kevin.tian@intel.com> wrote:

> Clear whole page when VMCS is reused due to cpu hotplug.
> construct_vmcs doesn't take care of all fields and some stale
> content may make hardware unhappy.
> 
> Signed-off-by Kevin Tian <kevin.tian@intel.com>

Which hotplug would that be? I can't see how vmx_create_vmcs() can be called
more than once for any vcpu. It's called during vcpu creation and that's it
afaics. The current check for v->arch.hvm_vmx.vmcs==NULL is therefore
redundant.

 -- Keir

RE: [PATCH] Clear VMCS if reusing after vcpu reset

[Tian, Kevin]

>From: Keir Fraser [mailto:Keir.Fraser@cl.cam.ac.uk]
>Sent: 2007年4月30日 14:41
>On 30/4/07 07:09, "Tian, Kevin" <kevin.tian@intel.com> wrote:
>
>> Clear whole page when VMCS is reused due to cpu hotplug.
>> construct_vmcs doesn't take care of all fields and some stale
>> content may make hardware unhappy.
>>
>> Signed-off-by Kevin Tian <kevin.tian@intel.com>
>
>Which hotplug would that be? I can't see how vmx_create_vmcs() can
>be called
>more than once for any vcpu. It's called during vcpu creation and that's it
>afaics. The current check for v->arch.hvm_vmx.vmcs==NULL is
>therefore
>redundant.
>
> -- Keir

Within HVM domain:
Echo "0" > /sys/devices/system/cpu/cpuN/online;
Echo "1" > /sys/devices/system/cpu/cpuN/online; 

Hvm_vcpu_reset is invoked as the result of receiving vINIT, which 
then goes to hvm_vcpu_initialise, vmx_vcpu_initialise, 
vmx_create_vmcs.

Thanks,
Kevin

RE: [PATCH] Clear VMCS if reusing after vcpu reset

[Li, Xin B]

>>Which hotplug would that be? I can't see how vmx_create_vmcs() can be
called
>>more than once for any vcpu. It's called during vcpu creation and
that's it
>>afaics. The current check for v->arch.hvm_vmx.vmcs==NULL is therefore
>>redundant.
>>


After Xen 3.0.5, renaming is needed :-)


>Within HVM domain:
>Echo "0" > /sys/devices/system/cpu/cpuN/online;
>Echo "1" > /sys/devices/system/cpu/cpuN/online; 
>
>Hvm_vcpu_reset is invoked as the result of receiving vINIT, which 
>then goes to hvm_vcpu_initialise, vmx_vcpu_initialise, vmx_create_vmcs.
>

Re: [PATCH] Clear VMCS if reusing after vcpu reset

[Keir Fraser]

On 30/4/07 07:45, "Tian, Kevin" <kevin.tian@intel.com> wrote:

> Within HVM domain:
> Echo "0" > /sys/devices/system/cpu/cpuN/online;
> Echo "1" > /sys/devices/system/cpu/cpuN/online;
> 
> Hvm_vcpu_reset is invoked as the result of receiving vINIT, which
> then goes to hvm_vcpu_initialise, vmx_vcpu_initialise,
> vmx_create_vmcs.

Ah. Ok. :-)

 -- Keir

Re: [PATCH] Clear VMCS if reusing after vcpu reset

[Keir Fraser]

On 30/4/07 07:09, "Tian, Kevin" <kevin.tian@intel.com> wrote:

> Clear whole page when VMCS is reused due to cpu hotplug.
> construct_vmcs doesn't take care of all fields and some stale
> content may make hardware unhappy.

You almost certainly need to vmx_clear_vmcs() before you zap it, as there's
no reason it might not be active on some CPU when vmx_create_vmcs() is
called (domain_pause() does not deactivate VMCSs).

Furthermore, this need to zap the page a second time is worrying. We only
modify architectural state in the VMCS in a well-defined way. That shouldn't
upset the hardware. Is this actually an indication that there is some
architectural state that we are forgetting to re-initialise? If so then
that's the bug which should be fixed.

 -- Keir

RE: [PATCH] Clear VMCS if reusing after vcpu reset

[Tian, Kevin]

>From: Keir Fraser [mailto:Keir.Fraser@cl.cam.ac.uk]
>Sent: 2007年4月30日 15:12
>
>On 30/4/07 07:09, "Tian, Kevin" <kevin.tian@intel.com> wrote:
>
>> Clear whole page when VMCS is reused due to cpu hotplug.
>> construct_vmcs doesn't take care of all fields and some stale
>> content may make hardware unhappy.
>
>You almost certainly need to vmx_clear_vmcs() before you zap it, as
>there's
>no reason it might not be active on some CPU when vmx_create_vmcs()
>is
>called (domain_pause() does not deactivate VMCSs).

You're exactly right. :-)

>
>Furthermore, this need to zap the page a second time is worrying. We
>only
>modify architectural state in the VMCS in a well-defined way. That
>shouldn't
>upset the hardware. Is this actually an indication that there is some
>architectural state that we are forgetting to re-initialise? If so then
>that's the bug which should be fixed.
>

Does construct_vmcs need to explicitly care all fields if hardware 
just expects a zero there which can be satisfied by memset()? Anyway, 
I'll take a deeper look to check which unexpected fields are with stale 
content before reset.

Thanks,
Kevin

Re: [PATCH] Clear VMCS if reusing after vcpu reset

[Keir Fraser]

On 30/4/07 08:19, "Tian, Kevin" <kevin.tian@intel.com> wrote:

>> Furthermore, this need to zap the page a second time is worrying. We
>> only
>> modify architectural state in the VMCS in a well-defined way. That
>> shouldn't
>> upset the hardware. Is this actually an indication that there is some
>> architectural state that we are forgetting to re-initialise? If so then
>> that's the bug which should be fixed.
> 
> Does construct_vmcs need to explicitly care all fields if hardware
> just expects a zero there which can be satisfied by memset()? Anyway,
> I'll take a deeper look to check which unexpected fields are with stale
> content before reset.

Well, we actually write lots of zeroes in construct_vmcs(), so it's not like
we really take advantage of the memset(). And in fact strictly speaking I
don't think we can -- the only architectural interface to the VMCS is via
the stupid VMREAD/VMWRITE instructions. We can make no assumptions about
VMCS layout nor, presumably, field encodings (you might choose to write 1s
for 0s and 0s for 1s in the actual in-memory format, for all I know!).

 -- Keir

RE: [PATCH] Clear VMCS if reusing after vcpu reset

[Tian, Kevin]

>From: Keir Fraser [mailto:Keir.Fraser@cl.cam.ac.uk]
>Sent: 2007年4月30日 15:25
>
>Well, we actually write lots of zeroes in construct_vmcs(), so it's not like
>we really take advantage of the memset(). And in fact strictly speaking I
>don't think we can -- the only architectural interface to the VMCS is via
>the stupid VMREAD/VMWRITE instructions. We can make no
>assumptions about
>VMCS layout nor, presumably, field encodings (you might choose to
>write 1s
>for 0s and 0s for 1s in the actual in-memory format, for all I know!).
>
> -- Keir

Agree.

Thanks,
Kevin

question about blktap

[tgh]

hi
I am using blktap,and I want to understand how does the blktap work
In the domU,there seems to be no blktap in it ,is it right?
If it is, does an application in domU access to the disk through the
frontend in domU and blktap in dom0 and then backend in dom0,or does it
just walk through frontend in domU and blktap in dom0, or how does it work?

could you help me
Thanks in advance