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Re: [PATCH] x86/nsvm: Fix L2 VINTR intercept merging when only Xen intercepts L1
- To: ross.lagerwall@xxxxxxxxxx
- From: Stephen Cheng <stephen.cheng@xxxxxxxxxx>
- Date: Fri, 6 Nov 2026 11:27:25 +0800
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- Cc: abdelkareem.abdelsaamad@xxxxxxxxxx, andrew.cooper3@xxxxxxxxxx, jason.andryuk@xxxxxxx, jbeulich@xxxxxxxx, roger@xxxxxxxxxxxxxx, teddy.astie@xxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx, Stephen Cheng <stephen.cheng@xxxxxxxxxx>
- Delivery-date: Fri, 06 Nov 2026 03:29:44 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 05/11/2026 14:16, Ross Lagerwall wrote:
> If L0 wants to inject an interrupt but the VINTR intercept is not enabled, how
> would it detect when it can inject the interrupt? In particular, if VMCB(1-2)
> has V_INTR_MASKING=0 and L2 sets EFLAGS.IF=1, the interrupt should be
> delivered but L0 won't notice.
Thanks, you are right. I missed this case.
But I think the current code does not handle this case either. L0's
dummy V_IRQ is not copied into VMCB(0-2). The V_IRQ in VMCB(0-2) comes
from VMCB(1-2). So the merged VINTR intercept only fires when L1 itself
has a V_IRQ pending for L2. If L1 has nothing pending, L0 also does not
notice when L2 sets EFLAGS.IF today. And when the intercept does fire,
the VMEXIT_VINTR handler clears L1's V_IRQ. This is the bug this patch
wants to fix.
My suggestion for this patch is to only remove the intercept when
ns_hostflags.vintrmask && !ns_hostflags.rflagsif. In this case no L1
interrupt can be delivered while L2 runs, so L0 does not need the
window. For the other cases, Xen needs to put its dummy V_IRQ into
VMCB(0-2) together with the intercept, and restore L1's V_IRQ later if
L1 had one.
Please drop my Reviewed-by for now.
Stephen
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