Re: [PATCH 1/2] KVM: nSVM: Restore L1's debug registers on nested VM-Exit

[Hao Zhang <hao_zhang_kdev@163.com>]

On Thu, Aug 27, 2026, Sean Christopherson wrote:
> On Thu, Aug 27, 2026, Hao Zhang wrote:
> > From: Hao Zhang <zhanghao1@kylinos.cn>
> > 
> > Restore L1's DR6 and DR7 when emulating a nested VM-Exit from L2 to L1.
> > KVM loads L2's debug register state from vmcb12 on nested VMRUN, but
> > currently restores DR7 to its reset value and leaves DR6 untouched when
> > switching back to vmcb01.  As a result, L1 can observe stale L2 DR6 state
> > and lose its DR7 state after an L2 VM-Exit.
> 
> Well, yeah, because the APM says so.  The APM very clearly says:
> 
>   Disables all breakpoints in the host DR7 register.
> 
> In typical APM fashion, it says nothing about DR6, so it's reasonable to assume
> it's preserved as-is across #VMEXIT, i.e. is L1's responsibility to ignore since
> there's no #DB.
>
Thanks. I checked the APM which requires #VMEXIT disables all breakpoints in
the host DR7 register, which means KVM's DR7_FIXED_1 restore on nested VM-Exit 
is intentional architectural behavior for L1-as-host, not a lost-state bug.

I'll drop this patch as-is.

Thanks,
Hao
> > Restore both registers from vmcb01, matching the rest of the L1 processor
> > state restored on nested VM-Exit.
> 
> What happens to other state is irrelevant, what matters is what the architecture
> says happens.