Re: About support for memory event on AMD CPUs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 24.11.2021 10:23, Untitled YAN wrote:
> Original email:
> 
> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: Wednesday, November 24, 2021 15:54
> To: YAN Untitled <yanluyi2010@xxxxxxxxxxx>
> Cc: tamas@xxxxxxxxxxxxx <tamas@xxxxxxxxxxxxx>; aisaila@xxxxxxxxxxxxxxx 
> <aisaila@xxxxxxxxxxxxxxx>; ppircalabu@xxxxxxxxxxxxxxx 
> <ppircalabu@xxxxxxxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx 
> <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> Subject: Re: About support for memory event on AMD CPUs 
>  
>> On 23.11.2021 18:14, YAN Untitled wrote:
>>> Dear developers,
>>>
>>> Short version: is memory event supported on AMD CPUs or is it going to be
>>> supported?
>>>
>>> Long version:
>>>
>>> Environment: LibVMI 0.15.0 + Xen 4.14.3 on an **AMD CPU** (5950x), 64-bit
>>> Debian 11 Dom0.
>>>
>>> What I am trying to accomplish: register a **memory event** handler, so that
>>> I can capture all memory accesses within a specific range issued by any 
>>> thread,
>>> or all memory accesses within any range issued by a specific thread.
>>>
>>> What I got instead: an error from LibVMI saying
>>> "xc_hvm_set_mem_access failed with code: -1".
>>>
>>> Some investigation: by inspecting the source code of LibVMI, I find the 
>>> direct
>>> cause is one of the libxc functions, 1) xc_set_mem_access or
>>> 2) xc_altp2m_set_mem_access, returned error code -1.
>>>
>>> After some searching, I found someone else having a similar problem [1]. I 
>>> also
>>> noted LibVMI says:
>>>
>>>> Currently only the Xen Hypervisor provides these features,
>>>> and some of these are specifically only available on Intel CPUs
>>>
>>> However, I can't find the exact confirmation for the availability of memory
>>> event on AMD CPUs from https://wiki.xenproject.org.
>>
>> Aiui underlying what you want is altp2m, which presently depends (in
>> the hypervisor) on EPT being available (and in use for the guest in
>> question).
>>
>> Jan
> 
> Dear Jan,
> 
> Thanks for your reply. I really appreciate the information. After some
> searching with the keyword "EPT" I found [1]. So I checked on dom0 with
> "xl dmesg | grep HAP", which says:
> 
>> (XEN) HVM: Hardware Assisted Paging (HAP) detected
>> (XEN) HVM: HAP page sizes: 4kB, 2MB, 1GB
> 
> And the guest is an HVM guest running Debian 10 with kernel v4.16 built from
> source. Most of the configuration of the guest is generated by virt-manager.
> The "xl list -l" shows that there is no explicit configuration for EPT, RVI or
> HAP, but it does say the "type" is "hvm".

HAP is the term covering EPT on the Intel side and NPT for AMD.

Jan