On Thu, Sep 20, 2018 at 04:02:43PM -0700, Jim Mattson wrote:
> Per volume 2 of the SDM, bits 63:15 and 12:4 of the exit qualification
> field for debug exceptions are reserved (cleared to 0).
Ugh, I suspect we have a documentation bug regarding bit 16 (DR6.BTM),
and maybe bit 15 (DR6.BT) as well. I'm pretty sure DR6.BTM should be
preserved if either HLE or RTM is supported. DR6.BT will never be set,
but that's because it's impossible to take a task-switch #DB in guest
due to task switches unconditionally taking VMExit and not because the
bit is explicitly cleared. DR6.BT doesn't really matter but I'll ask
since I need to bug someone about DR6.BTM anyways.
Maybe just explicitly clear DR6_FIXED_1 for now? DR6_BT should be a
non-issue. DR6.BTM is weird because of its inverted behavior, not
sure what the safest approach for that bit would be.
P.S. I assume you meant Volume 3?
> There is still an issue with stale DR6 bits potentially being
> misreported for the current debug exception. DR6 should not have been
> modified before vectoring the #DB exception, and the "new DR6 bits"
> should be available somewhere, but it was and they aren't.
>
> Fixes: b96fb439774e1 ("KVM: nVMX: fixes to nested virt interrupt injection")
> Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
> ---
> arch/x86/include/asm/kvm_host.h | 1 +
> arch/x86/kvm/vmx.c | 3 ++-
> 2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 8e90488c3d56..234f7320ba59 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -177,6 +177,7 @@ enum {
>
> #define DR6_BD (1 << 13)
> #define DR6_BS (1 << 14)
> +#define DR6_BT (1 << 15)
> #define DR6_RTM (1 << 16)
> #define DR6_FIXED_1 0xfffe0ff0
> #define DR6_INIT 0xffff0ff0
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 533a327372c8..01f729830fd9 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3293,7 +3293,8 @@ static int nested_vmx_check_exception(struct kvm_vcpu *vcpu, unsigned long *exit
> } else {
> if (vmcs12->exception_bitmap & (1u << nr)) {
> if (nr == DB_VECTOR)
> - *exit_qual = vcpu->arch.dr6;
> + *exit_qual = vcpu->arch.dr6 &
> + ~(DR6_FIXED_1 | DR6_BT | DR6_RTM);
>
> else
> *exit_qual = 0;
> return 1;
> --
> 2.19.0.444.g18242da7ef-goog
>
