linux-kernel - Re: [PATCH 0/3] KVM: x86: cleanup and fixes for debug register accesses

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Date:   Mon, 4 May 2020 21:20:05 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH 0/3] KVM: x86: cleanup and fixes for debug register
 accesses

On 04/05/20 20:55, Peter Xu wrote:
> On Mon, May 04, 2020 at 11:55:55AM -0400, Paolo Bonzini wrote:
>> The purpose of this series is to get rid of the get_dr6 accessor
>> and, on Intel, of set_dr6 as well.  This is done mostly in patch 2,
>> since patch 3 is only the resulting cleanup.  Patch 1 is a related
>> bug fix that I found while inspecting the code.
> 
> Reviewed-by: Peter Xu <peterx@...hat.com>
> 
> (Btw, the db_interception() change in patch 2 seems to be a real fix to me)

It should be okay because vcpu->arch.dr6 is not used on AMD.

However I think a kvm_update_dr6 call is missing in
kvm_deliver_exception_payload, and kvm_vcpu_check_breakpoint should use
kvm_queue_exception_p.  I'll fix all of those.

> I have that in my list, but I don't know it's "sorely" needed. :) It was low
> after I knew the fact that we've got one test in kvm-unit-test, but I can for
> sure do that earlier.
> 
> I am wondering whether we still want a test in selftests if there's a similar
> test in kvm-unit-test already.  For this one I guess at least the guest debug
> test is still missing.

The guest debugging test would basically cover the gdbstub case, which
is different from kvm-unit-tests.  It would run similar tests to
kvm-unit-tests, but #DB and #BP exceptions would be replaced by
KVM_EXIT_DEBUG, and MOVs to DR would be replaced by KVM_SET_GUEST_DEBUG.

It could also cover exception payload support in KVM_GET_VCPU_EVENTS,
but that is more complicated because it would require support for
exceptions in the selftests.

Paolo