AMD NPT support for Xen memory events

AMD NPT support for Xen memory events

[Abhiroop Dabral]

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Hi,

I was looking at the code for the memory events for guest domain in
"xen/arch/xen/mm/mem_event.c". In the function "mem_event_domctl()" for the
case "XEN_DOMCTL_MEM_EVENT_OP_ACCESS", there is a comment that "Currently
only EPT is supported" and then a check is made for the vmx presence and
control returns if it is not Intel VT with an error.
I am trying to make the memory events working on AMD SVM with NPT support.
My question is that is there a specific reason why AMD NPT is supported
there?
The things I should keep in mind/work on to support the memory events on
AMD NPT.
(Working on Xen 4.4 on HVM guests (windows XP 32 bit -PAE)

thanks

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Re: AMD NPT support for Xen memory events

[Andrew Cooper]

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On 16/07/14 05:38, Abhiroop Dabral wrote:
> Hi,
>
> I was looking at the code for the memory events for guest domain in
> "xen/arch/xen/mm/mem_event.c". In the function "mem_event_domctl()"
> for the case "XEN_DOMCTL_MEM_EVENT_OP_ACCESS", there is a comment that
> "Currently only EPT is supported" and then a check is made for the vmx
> presence and control returns if it is not Intel VT with an error. 
> I am trying to make the memory events working on AMD SVM with NPT
> support. 
> My question is that is there a specific reason why AMD NPT is
> supported there?
> The things I should keep in mind/work on to support the memory events
> on AMD NPT.
> (Working on Xen 4.4 on HVM guests (windows XP 32 bit -PAE)

I am not aware of any technical reasons why AMD support could not be added.

I don't know why there is still a VMX check.  c/s d928515d76 took out an
EPT check and that appears to have been fine.

Try it and see - it could be as easy as just deleting the vmx check.

It would certainly be nice to try and get some parity back in our
feature sets; a number of new features arrive as Intel-only because they
were developed on Intel systems, rather than a specific reason why the
feature wouldn't work on AMD.

~Andrew

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Re: AMD NPT support for Xen memory events

[Aravindh Puthiyaparambil (aravindp)]

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I was looking at the code for the memory events for guest domain in "xen/arch/xen/mm/mem_event.c". In the function "mem_event_domctl()" for the case "XEN_DOMCTL_MEM_EVENT_OP_ACCESS", there is a comment that "Currently only EPT is supported" and then a check is made for the vmx presence and control returns if it is not Intel VT with an error.
I am trying to make the memory events working on AMD SVM with NPT support.
My question is that is there a specific reason why AMD NPT is supported there?
The things I should keep in mind/work on to support the memory events on AMD NPT.
(Working on Xen 4.4 on HVM guests (windows XP 32 bit -PAE)

I am not aware of any technical reasons why AMD support could not be added.

I don't know why there is still a VMX check.  c/s d928515d76 took out an EPT check and that appears to have been fine.

Try it and see - it could be as easy as just deleting the vmx check.

Deleting the vmx check might not be sufficient w.r.t mem_access. The AMD NPT structure differs from the Intel EPT structures. NPT layout is more like a regular page-table. Whereas EPT has explicit R, W, X bits allowing for access permissions like execute only i.e. a page that the guest can only execute but not read. A guest read will cause a mem_event to be fired to the listener. I am not sure if this is possible easily with AMD but definitely a sub-set of the permissions is definitely possible. This is what I did in my RFC series for enabling mem_access for PV domains.

It would certainly be nice to try and get some parity back in our feature sets; a number of new features arrive as Intel-only because they were developed on Intel systems, rather than a specific reason why the feature wouldn't work on AMD.

I second this and it would certainly be good to have feature parity across AMD and Intel processors.

Thanks,
Aravindh

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