Re: [PATCH] kvm/svm: fix DR interception handling on upcoming AMD CPUs

[Avi Kivity <avi@redhat.com>]

On 02/09/2011 02:56 PM, Roedel, Joerg wrote:
> On Wed, Feb 09, 2011 at 05:43:28AM -0500, Andre Przywara wrote:
> >  Roedel, Joerg wrote:
> >  >  On Tue, Feb 08, 2011 at 07:22:29PM -0500, Andre Przywara wrote:
> >  >>  Somehow the code line advancing the RIP and checking for exceptions
> >  >>  got dropped between the post on the ML and the commit.
> >  >>  Add it again to let guests boot on upcoming AMD CPUs again.
> >  >>
> >  >>  Reported-by: Joerg Roedel<joerg.roedel@amd.com>
> >  >>  Signed-off-by: Andre Przywara<andre.przywara@amd.com>
> >  >>  ---
> >  >>   arch/x86/kvm/svm.c |    1 +
> >  >>   1 files changed, 1 insertions(+), 0 deletions(-)
> >  >>
> >  >>  diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> >  >>  index 25bd1bc..be06e58 100644
> >  >>  --- a/arch/x86/kvm/svm.c
> >  >>  +++ b/arch/x86/kvm/svm.c
> >  >>  @@ -2776,6 +2776,7 @@ static int dr_interception(struct vcpu_svm *svm)
> >  >>   		if (!err)
> >  >>   			kvm_register_write(&svm->vcpu, reg, val);
> >  >>   	}
> >  >>  +	kvm_complete_insn_gp(&svm->vcpu, err);
> >  >
> >  >  Any exception check takes presedence over the intercept. So a call to
> >  >  skip_emulated_instruction should be sufficient here.
>
> >  You are right, but I think this way the implementation is cleaner and
> >  more generic. If there is no exception condition detected (err==0), then
> >  the call will just do skip_emulated_instruction anyway.
> >  But as I write this, I see that err is not initialized, so I'd propose
> >  this corrected version of the patch instead.
>
> One function call is not less clean than another and this function call
> has more overhead.
> Beside that, looking at kvm_set_dr this function only returns 1 if one
> tries to set dr4 or dr5 while CR4.DE is not set. In this case the
> function you call here would inject a #GP but architecturally a #UD
> is required.
> This problem doesn't matter too much because the exceptions take
> precedence over the intercept anyway so it is sufficient to call
> skip_emulated_instruction here.

Given that the exception is incorrect I agree 
skip_emulated_instruction() is a better choice here.

-- 
error compiling committee.c: too many arguments to function