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Re: [PATCH v2 02/13] nestedsvm: Adjust L2's DR intercept when adjusting L1
- To: chunjie.zhu@xxxxxxxxxx, ross.lagerwall@xxxxxxxxxx
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 13 Oct 2026 12:15:15 +0200
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- Cc: andrew.cooper3@xxxxxxxxxx, roger@xxxxxxxxxxxxxx, jason.andryuk@xxxxxxx, teddy.astie@xxxxxxxxxx, stephen.cheng@xxxxxxxxxx, lin.liu01@xxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 13 Oct 2026 10:15:21 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 10.10.2026 12:13, chunjie.zhu@xxxxxxxxxx wrote:
> From: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
>
> If L2 accesses a debug register (like reading DR7) without L1 intercepting
> it, it locks up the vCPU. L0 intercepts VMEXIT_DR7_READ, which disables
> the intercept for L1 and then restarts L2 which re-executes the
> instruction and then this repeats indefinitely.
>
> Disable the intercept for the current VMCB if in guest mode to reflect
> what would happen if the VMCB were recreated via
> nsvm_vmcb_prepare4vmrun().
>
> Fixes: a59a7be91b61 ("nestedsvm: fix DRn handling")
> Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
Andrew's comment [1] was left unaddressed. Comments on earlier versions
should be dealt with, one way or another, before re-submitting.
Jan
> --- a/xen/arch/x86/hvm/svm/svm.c
> +++ b/xen/arch/x86/hvm/svm/svm.c
> @@ -1657,6 +1657,8 @@ static void svm_dr_access(struct vcpu *v, struct
> cpu_user_regs *regs)
>
> TRACE(TRC_HVM_DR_WRITE);
> __restore_debug_registers(vmcb, v);
> + if ( nestedhvm_enabled(v->domain) && nestedhvm_vcpu_in_guestmode(v) )
> + vmcb_set_dr_intercepts(v->arch.hvm.svm.vmcb, 0);
> }
>
> static int cf_check svm_msr_read_intercept(
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