Re: [PATCH v2 2/2] x86/nestedsvm: honour L2's EFLAGS.IF when L1 clears V_INTR_MASKING

[Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>]

On 11/10/26 8:57 AM, chunjie.zhu@xxxxxxxxxx wrote:
> From: Chunjie Zhu <chunjie.zhu@xxxxxxxxxx>
>
> L0 always runs L2 with V_INTR_MASKING set, so a physical interrupt exits to
> L0 regardless of L2's EFLAGS.IF.  nsvm_intr_blocked() only blocks when L1
> requested V_INTR_MASKING=1 and L1's IF was clear.  With V_INTR_MASKING=0
> and L2's IF clear, nestedsvm_vcpu_interrupt() would still inject
> VMEXIT(INTR) into L1.
>
> Per the APM (vol.2 rev 3.44 "Physical Interrupt Masking in EFLAGS"), with
> V_INTR_MASKING=0 L2's IF masks physical interrupts, and a masked interrupt
> must stay pending.  Return NSVM_INTR_MASKED for external interrupts in
> that case.
>
> Signed-off-by: Chunjie Zhu <chunjie.zhu@xxxxxxxxxx>
> ---
> v2:
>   * Move the check into nsvm_intr_blocked(), as suggested by Ross.
>
>   xen/arch/x86/hvm/svm/nestedsvm.c | 12 ++++++++++--
>   1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/xen/arch/x86/hvm/svm/nestedsvm.c b/xen/arch/x86/hvm/svm/nestedsvm.c
> index a93681e9bbcd..e3b2d20ffce2 100644
> --- a/xen/arch/x86/hvm/svm/nestedsvm.c
> +++ b/xen/arch/x86/hvm/svm/nestedsvm.c
> @@ -1222,8 +1222,16 @@ enum hvm_intblk cf_check nsvm_intr_blocked(struct vcpu 
> *v)
>       {
>           struct vmcb_struct *n2vmcb = nv->nv_n2vmcx;
>   
> -        if ( svm->ns_hostflags.fields.vintrmask &&
> -             !svm->ns_hostflags.fields.rflagsif )
> +        /*
> +         * With V_INTR_MASKING, L1's EFLAGS.IF masks physical interrupts.
> +         * Without it, L2's EFLAGS.IF does (APM vol.2 rev 3.44 "Physical
> +         * Interrupt Masking in EFLAGS"); L0 always runs L2 with
> +         * V_INTR_MASKING set, so the interrupt would otherwise exit to L0
> +         * and be forwarded to L1 even with L2's IF clear.
> +         */
> +        if ( svm->ns_hostflags.fields.vintrmask
> +             ? !svm->ns_hostflags.fields.rflagsif
> +             : !(guest_cpu_user_regs()->eflags & X86_EFLAGS_IF) )
>               return hvm_intblk_rflags_ie;
>   
>           /* when l1 guest passes its devices through to the l2 guest
This only works if the function is called with v == current. I think it
is, but it should probably have an assert to confirm this (like
hvm_interrupt_blocked()).

Also note that this series needs rebasing on top of the latest staging
since it doesn't apply cleanly.

Ross