Re: [PATCH v2 1/2] x86/traps: Initialize DR6 by writing its architectural reset value

[Xin Li]

On 6/18/2025 2:24 PM, Sean Christopherson wrote:
> On Tue, Jun 17, 2025, Xin Li wrote:
> > On 6/17/2025 1:47 PM, Sean Christopherson wrote:
> > > On Tue, Jun 17, 2025, Sohil Mehta wrote:
> > > Note, DR6_VOLATILE and DR6_FIXED_1 aren't necessarily aligned with the current
> > > architectural definitions (I haven't actually checked),
> >
> > I'm not sure what do you mean by "architectural definitions" here.
>
> I was trying to say that there may be bits that have been defined in the SDM,
> but are not yet makred as "supported" in DR6_VOLATILE, i.e. that are "incorrectly"
> marked as DR6_FIXED_1 (in quotes, because from KVM's perspective, the bits *are*
> fixed-1, for the guest).

Clear for me now, thanks for the explanation.

Wanted to echo that it's necessary to be in a KVM mindset sometimes.

>   
> > However because zeroing DR6 leads to different DR6 values depending on
> > whether the CPU supports BLD:
> >
> >    1) On CPUs with BLD support, DR6 becomes 0xFFFF07F0 (bit 11, DR6.BLD,
> >       is cleared).
> >
> >    2) On CPUs without BLD, DR6 becomes 0xFFFF0FF0.
> >
> > DR6_FIXED_1, if it is still defined to include all bits that can't be
> > cleared, is a constant value only on a *specific* CPU architecture,
> > i.e., it is not a constant value on all CPU implementations.
> >
> >
> > > rather they are KVM's
> > > view of the world, i.e. what KVM supports from a virtualization perspective.
> >
> > So KVM probably should expose the fixed 1s in DR6 to the guest depending on
> > which features, such as BLD or RTM, are enabled and visible to the
> > guest or not?
> >
> > (Sorry I haven't looked into how the macro DR6_FIXED_1 is used in KVM,
> > maybe it's already used in such a way)
>
> Yep, that's exactly what KVM does.  DR6_FIXED_1 is the set of bits that KVM
> doesn't yet support for *any* guest.  The per-vCPU set of a fixed-1 bits starts
> with DR6_FIXED_1, and adds in bits for features that aren't supported/exposed
> to the guest.
>
> static u64 kvm_dr6_fixed(struct kvm_vcpu *vcpu)
> {
> 	u64 fixed = DR6_FIXED_1;
>
> 	if (!guest_cpu_cap_has(vcpu, X86_FEATURE_RTM))
> 		fixed |= DR6_RTM;
>
> 	if (!guest_cpu_cap_has(vcpu, X86_FEATURE_BUS_LOCK_DETECT))
> 		fixed |= DR6_BUS_LOCK;
> 	return fixed;
> }

Excellent!

KVM is aware of BLD and behaves the same as real hardware, doing a
better job than native regarding this specific case.