Re: [PATCH v2 for-4.14 1/2] x86/mem_sharing: block interrupt injection for forks

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.05.2020 14:18, Tamas K Lengyel wrote:
> On Mon, May 25, 2020 at 12:06 AM Jan Beulich <jbeulich@xxxxxxxx> wrote:
>>
>> On 22.05.2020 18:33, Tamas K Lengyel wrote:
>>> When running shallow forks without device models it may be undesirable for 
>>> Xen
>>> to inject interrupts. With Windows forks we have observed the kernel going 
>>> into
>>> infinite loops when trying to process such interrupts, likely because it 
>>> attempts
>>> to interact with devices that are not responding without QEMU running. By
>>> disabling interrupt injection the fuzzer can exercise the target code 
>>> without
>>> interference.
>>>
>>> Forks & memory sharing are only available on Intel CPUs so this only applies
>>> to vmx.
>>
>> Looking at e.g. mem_sharing_control() I can't seem to be able to confirm
>> this. Would you mind pointing me at where this restriction is coming from?
> 
> Both mem_access and mem_sharing are only implemented for EPT:
> http://xenbits.xen.org/hg/xen-unstable.hg/file/5eadf9363c25/xen/arch/x86/mm/p2m-ept.c#l126.

p2m-pt.c:p2m_type_to_flags() has a similar case label. And I can't
spot a respective restriction in mem_sharing_memop(), i.e. it looks
to me as if enabling mem-sharing on NPT (to satisfy hap_enabled()
in mem_sharing_control()) would be possible.

Jan