[Xen-devel] Question about intercept debug breakpoint in the Guest OS

[Kai Luo <kluo@xxxxxxxxxx>]

Hi:

    Recently I try to intercept a given kernel address-syscall address,for example- of the guest windows 7 to the hypervisor, I set the exception flag in the EXCEPTION_BITMAP as follows:

vmcs.c

void vmx_do_resume(vcpu * v)

{

......

        unsigned long intercepts = __vmread(EXCEPTION_BITMAP);

        unsigned long mask = (1UL << TRAP_debug);

        intercepts |= mask;

        __vmwrite(EXCEPTION_BITMAP, intercepts) 

        v->arch.guest_context.debugreg[0] = [Address of a syscall];

        v->arch.guest_context.debugreg[7] |= 2;

......

}

I do reveive the VMexit in the vmx_vmexit_handler in vmx.c  then I want  to let the guest os keep going as usual without concerning about the debug breakpoint so I set some flags as follows:

case TRAP_debug:

......

    regs->eflags |= X86_EFLAGS_RF;

    exit_qualification = __vmread(EXIT_QUALIFICATION);

    write_debug(6, exit_qualification | 0xffff0ff0);

......

The problem is the guest os seems trapped into an infinite loop and take the full vcpu capacity,  I find the RIP in the VCPU stay the same, EFLAGS value of the VCPU is 0x2 and the guest os can't respond to any movement of  mouse or keyboard,.

        Do I missed some flag when restoring the guest context?Or there are some other reasons that lead to this issue?Could you give me any suggestions?

Jone

......

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