From: Paolo Bonzini <pbonzini@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH 9/9] KVM: VMX: pass correct DR6 for GD userspace exit
Date: Thu, 7 May 2020 19:42:25 +0200 [thread overview]
Message-ID: <db06ffa7-1e3c-14e5-28b8-5053f4383ecf@redhat.com> (raw)
In-Reply-To: <20200507163839.GG228260@xz-x1>
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.
next prev parent reply other threads:[~2020-05-07 17:42 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-07 11:50 [PATCH v2 0/9] KVM_SET_GUEST_DEBUG tests and fixes, DR accessors cleanups Paolo Bonzini
2020-05-07 11:50 ` [PATCH v2 1/9] KVM: X86: Declare KVM_CAP_SET_GUEST_DEBUG properly Paolo Bonzini
2020-05-07 11:50 ` [PATCH v2 2/9] KVM: x86: fix DR6 delivery for various cases of #DB injection Paolo Bonzini
2020-05-07 11:50 ` [PATCH v2 3/9] KVM: X86: Set RTM for DB_VECTOR too for KVM_EXIT_DEBUG Paolo Bonzini
2020-05-07 11:50 ` [PATCH v2 4/9] KVM: X86: Fix single-step with KVM_SET_GUEST_DEBUG Paolo Bonzini
2020-05-07 11:50 ` [PATCH v2 5/9] KVM: selftests: Add KVM_SET_GUEST_DEBUG test Paolo Bonzini
2020-05-07 11:50 ` [PATCH v2 6/9] KVM: nSVM: trap #DB and #BP to userspace if guest debugging is on Paolo Bonzini
2020-05-07 18:22 ` Peter Xu
2020-05-07 11:50 ` [PATCH v2 7/9] KVM: SVM: keep DR6 synchronized with vcpu->arch.dr6 Paolo Bonzini
2020-05-07 18:22 ` Peter Xu
2020-05-07 22:21 ` Paolo Bonzini
2020-05-07 11:50 ` [PATCH v2 8/9] KVM: x86, SVM: isolate vcpu->arch.dr6 from vmcb->save.dr6 Paolo Bonzini
2020-05-07 19:28 ` Peter Xu
2020-05-07 22:33 ` Paolo Bonzini
2020-05-08 15:32 ` Peter Xu
2020-05-09 13:28 ` Paolo Bonzini
2020-05-11 16:15 ` Peter Xu
2020-05-07 11:50 ` [PATCH 9/9] KVM: VMX: pass correct DR6 for GD userspace exit Paolo Bonzini
2020-05-07 16:18 ` Peter Xu
2020-05-07 16:21 ` Paolo Bonzini
2020-05-07 16:38 ` Peter Xu
2020-05-07 17:42 ` Paolo Bonzini [this message]
2020-05-07 18:05 ` Peter Xu
-- strict thread matches above, loose matches on Subject: below --
2020-05-06 11:10 [PATCH 0/9] KVM_SET_GUEST_DEBUG tests and fixes, DR accessors cleanups Paolo Bonzini
2020-05-06 11:10 ` [PATCH 9/9] KVM: VMX: pass correct DR6 for GD userspace exit Paolo Bonzini
2020-05-06 17:50 ` Peter Xu
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