[Xen-devel] Hypercall Inquiries

[Rapidash <Rapidash@xxxxxxxxxxxxxx>]

Greetings,

 My co-worker and I are looking into Xen Hypervisor. By any chance, do you or any of your colleagues have technical material/ papers/ presentations detailing how the hypercall interacts with the hypervisor?

 Thank you in advance for any assistance,

  - Rapidash

Some specific questions:

- If the memory of the VM is stored on non-congruent sections of the host's machine memory, how does the hypercall handler check whether a passed in pointer parameter falls within these VM claimed sections of memory?

- Since the hypercalls are limited in number compared to syscalls, is there ever an instance where the domain will require a syscall that the hypercall does not cover?

 

Also, we have been looking at the Xen hypercall source code to try and figure out the mechanics there. In file "hypercall-x86_64.h" there is the following code segment we are attempting to decipher (from it, we can figure out the preceding functions):

#define _hypercall5(type, name, a1, a2, a3, a4, a5)  \

({        \

 long __res, __ign1, __ign2, __ign3;   \

 asm volatile (      \

  "movq %7,%%r10; movq %8,%%r8; "   \

  "call hypercall_page + ("STR(__HYPERVISOR_##name)" * 32)"\

  : "=a" (__res), "=D" (__ign1), "=S" (__ign2), \

  "=d" (__ign3)     \

  : "1" ((long)(a1)), "2" ((long)(a2)),  \

  "3" ((long)(a3)), "g" ((long)(a4)),  \

  "g" ((long)(a5))    \

  : "memory", "r10", "r8" );   \

 (type)__res;      \

})

- The first line within the asm volatile section, are the contents of registers being saved to memory? If so, where?

- In the third and fourth lines within the same section, are values from __res being placed into the "a" register, or are the values within the "a" register being stored in the variable __res for use later?

- Does the "1", "2", "3", "g", "g" correspond to the ebx, ecx, edx, esi, and edi registers? Or are they a different set?

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