Re: [Xen-devel] [SVM] Getting the length of the current instruction in svm_vmexit_handler()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 14/03/18 15:53, Jan Beulich wrote:
>>>> On 14.03.18 at 15:56, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>> We'd like to retrieve the length of the current instruction in
>> svm_vmexit_handler(), specifically for the VMEXIT_EXCEPTION_DB and
>> VMEXIT_EXCEPTION_BP cases.
>>
>> We've combed the vmcb to no avail. Everything we've thought to check
>> (exitinfo1, exitinfo2, exitintinfo) turns out to be zero there while
>> testing.
>>
>> There's __get_instruction_length(vcpu, instr), but it expects to be fed
>> the exact instruction we want the length for, which obviously defeats
>> the purpose here.
>>
>> Is there a clean way to get the current instruction length like we do in
>> the VMX case (__vmread(VM_EXIT_INSTRUCTION_LEN, &insn_len)) that we're
>> overlooking?
> Just like Intel's, AMD's is available in a subset of cases only
> (look for vmcb->guest_ins_len), which don't include the
> exception intercepts you talk about. For #DB I think there's
> no difference between both anyway.

On non-first-gen hardware, the difference between RIP and NextRIP should
give you the instruction length.  ISTR NextRIP is written on all exits,
and consumed on all entries.

~Andrew

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