Re: [PATCH v5 1/2] KVM: nSVM: Raise #UD if unhandled VMMCALL isn't intercepted by L1

[Yosry Ahmed]

On Tue, Mar 03, 2026 at 04:22:22PM -0800, Sean Christopherson wrote:
> From: Kevin Cheng <chengkev@xxxxxxxxxx>
> 
> Explicitly synthesize a #UD for VMMCALL if L2 is active, L1 does NOT want
> to intercept VMMCALL, nested_svm_l2_tlb_flush_enabled() is true, and the
> hypercall is something other than one of the supported Hyper-V hypercalls.
> When all of the above conditions are met, KVM will intercept VMMCALL but
> never forward it to L1, i.e. will let L2 make hypercalls as if it were L1.
> 
> The TLFS says a whole lot of nothing about this scenario, so go with the
> architectural behavior, which says that VMMCALL #UDs if it's not
> intercepted.
> 
> Opportunistically do a 2-for-1 stub trade by stub-ifying the new API
> instead of the helpers it uses.  The last remaining "single" stub will
> soon be dropped as well.
> 
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Fixes: 3f4a812edf5c ("KVM: nSVM: hyper-v: Enable L2 TLB flush")
> Cc: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Kevin Cheng <chengkev@xxxxxxxxxx>
> Link: https://patch.msgid.link/20260228033328.2285047-5-chengkev@xxxxxxxxxx
> [sean: rewrite changelog and comment, tag for stable, remove defunct stubs]
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Yosry Ahmed <yosry@xxxxxxxxxx>