[Xen-devel] domain crashed when using VMFUNC

[liuweijie <liuwj0129@xxxxxxx>]

Dear list,

When I use VMFUNC instructions on a Xen HVM, domain crashes sometimes. 

My serial console shows like this:

domain_crash called from p2m.c:2204
Domain 1 (vcpu#0) crashed on cpu#7
……

My testbed runs on Xen-4.6.0, and my CPU is Intel i7-4790. I can provide more 
logs if needed.

I know you guys have implemented helpful interfaces to manage alternative P2Ms 
in version 4.6. Those ‘hvm_altp2m_op’ hypercalls are invoked before VMFUNC 
instructions are executed. And ten alternative P2Ms can be built successfully.

The pseudo-code of my experiment is as follows:

for (i = 0; i < 10; i++)
        switch the current eptp to eptp[i];


However, once switching to eptp[4], namely when doing "mov eax 0; mov ecx 4; 
vmfunc.”, my Ubuntu HVM crashes. And as soon as I switched to more than 4 
EPTPs, it crashed too. In other words, when I executed VMFUNC to switch to the 
fifth different altp2m, the domain would crash.

Then when I just created 4 altp2ms, that weird phenomenon never happened again. 
Four altp2ms seems tolerable, but I still would like to use more. In addition, 
the Intel manual says we can switch between 512 altp2ms, right?

FYI, I know the bug lies in the function ‘p2m_altp2m_lazy_copy’, and it is 
caused by the wrong return number of function ‘p2m_set_entry’.

Can you guys fix the bug? Or is there something wrong with my test?

Any help is appreciated! Thanks so much!

Cheers,
Weijie.

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