linux-kernel - Re: [PATCH 9/9] KVM: VMX: pass correct DR6 for GD userspace exit

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Date:   Thu, 7 May 2020 19:42:25 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 9/9] KVM: VMX: pass correct DR6 for GD userspace exit

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.