On 07/05/20 18:38, Peter Xu wrote:
> On Thu, May 07, 2020 at 06:21:18PM +0200, Paolo Bonzini wrote:
>> On 07/05/20 18:18, Peter Xu wrote:
>>>> if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP) {
>>>> - vcpu->run->debug.arch.dr6 = vcpu->arch.dr6;
>>>> + vcpu->run->debug.arch.dr6 = DR6_BD | DR6_RTM | DR6_FIXED_1;
>>> After a second thought I'm thinking whether it would be okay to have BS set in
>>> that test case. I just remembered there's a test case in the kvm-unit-test
>>> that checks explicitly against BS leftover as long as dr6 is not cleared
>>> explicitly by the guest code, while the spec seems to have no explicit
>>> description on this case.
>>
>> Yes, I noticed that test as well. But I don't like having different
>> behavior for Intel and AMD, and the Intel behavior is more sensible.
>> Also...
>
> Do you mean the AMD behavior is more sensible instead? :)
No, I mean within the context of KVM_EXIT_DEBUG: the Intel behavior is
to only include the latest debug exception in kvm_run's DR6 field, while
the AMD behavior would be to include all of them. This was an
implementation detail (it happens because Intel sets kvm_run's DR6 from
the exit qualification of #DB), but it's more sensible too.
In addition:
* AMD was completely broken until this week, so the behavior of
KVM_EXIT_DEBUG is defined de facto by kvm_intel.ko. Userspace has not
been required to set DR6 with KVM_SET_GUEST_DEBUG, and since we can
emulate that on AMD, we should.
* we have to fix anyway the fact that on AMD a KVM_EXIT_DEBUG is
clobbering the contents of the guest's DR6
>>> Intead of above, I'm thinking whether we should allow the userspace to also
>>> change dr6 with the KVM_SET_GUEST_DEBUG ioctl when they wanted to (right now
>>> iiuc dr6 from userspace is completely ignored), instead of offering a fake dr6.
>>> Or to make it simple, maybe we can just check BD bit only?
>>
>> ... I'm afraid that this would be a backwards-incompatible change, and
>> it would require changes in userspace. If you look at v2, emulating the
>> Intel behavior in AMD turns out to be self-contained and relatively
>> elegant (will be better when we finish cleaning up nested SVM).
>
> I'm still trying to read the other patches (I need some more digest because I'm
> even less familiar with nested...). I agree that it would be good to keep the
> same behavior across Intel/AMD. Actually that also does not violate Intel spec
> because the AMD one is stricter.
Again, careful---we're talking about KVM_EXIT_DEBUG, not the #DB exception.
Thanks,
Paolo
> However I guess then we might also want to
> fixup the kvm-unit-test too to aligh with the behaviors on leftover set bits.
