Re: [RFC PATCH] kvm: Make KVM DF intercept configurable

Jan Kiszka
2016-03-02T06:45:53Z

On 2016-03-01 23:41, Borislav Petkov wrote:
> On Tue, Mar 01, 2016 at 10:11:57PM +0100, Paolo Bonzini wrote:
>> I just use QEMU's binary translation mode to debug this kind of code (-d
>> in_asm is useful and relatively compact (because it only shows loops
>> once),
>
> Ha, I had forgotten about "in_asm"! Thanks for reminding me, that's a
> really cool feature I'm going to use. With it I see:
>
> ----------------
> IN:
> 0x0000000001000000: mov 0xffffffff81cba1f8,%rsp
> 0x0000000001000008: callq 0x10001a9
>
> ----------------
>
> Now, it is obvious that 0xffffffff81cba1f8 is not mapped yet and we're
> running from physical addresses. The DF tracepoint shows, in addition,
> the previous exception vector causing the DF and I think that's useful.
> As an additional debugging aid. Oh, and that doesn't need ept=0 and runs
> at full speed.
>
>> or alternatively ept=0. But perhaps... why not. :) It's not like it
>> adds overhead.
>
> Yeah, it is off by default and doesn't hurt anyone. And the diff size
> is ok, IMHO. Lemme code the Intel side too and see how the whole thing
> turns out.

To make this a serious debug feature, we should consider trapping all
exceptions on request (and reinjecting the unhandled ones). Not sure
right now, though, if that comes with more complications than the simple
#DF case.

Jan

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