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Re: [PATCH v2] x86/nestedsvm: remove dead code for shadow-paging L1 hosts
- To: chunjie.zhu@xxxxxxxxxx
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 27 Oct 2026 08:45:57 +0100
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- Cc: andrew.cooper3@xxxxxxxxxx, jason.andryuk@xxxxxxx, roger@xxxxxxxxxxxxxx, teddy.astie@xxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 27 Oct 2026 07:46:05 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 27.10.2026 08:37, chunjie.zhu@xxxxxxxxxx wrote:
> From: Chunjie Zhu <chunjie.zhu@xxxxxxxxxx>
>
> Nested virtualization on a domain is only permitted together with HAP:
> sanitise_domain_config() rejects XEN_DOMCTL_CDF_nested_virt requested
> without XEN_DOMCTL_CDF_hap, since shadow paging is too slow to be usable
> as the L0 paging mode underneath an L1 hypervisor. This means the host
> (L0) can never be in shadow paging mode while nested_virt is enabled,
> i.e. paging_mode_shadow(v->domain) is always false in this context.
>
> The nested SVM code still had incomplete/speculative handling for the
> 'host shadow paging + guest shadow paging' case in
> nsvm_vcpu_hostrestore(), nsvm_vmcb_prepare4vmrun(),
> nsvm_vmcb_prepare4vmexit() and nestedsvm_check_intercepts(). Remove it,
> and assert the host paging mode with a single
> BUG_ON(paging_mode_shadow(v->domain)) ahead of the guest paging mode
> checks, so the remaining comments only describe the guest mode.
>
> In nestedsvm_check_intercepts()'s #PF handling, fold the two identical
> NESTEDHVM_VMEXIT_INJECT returns.
>
> Also drop the now always-true paging_mode_hap() check from
> svm_nested_features_on_efer_update(), and the stale "shadow-on-shadow"
> TODO in paging_update_nestedmode().
>
> No functional change.
>
> Signed-off-by: Chunjie Zhu <chunjie.zhu@xxxxxxxxxx>
Two formal things only for now (will look at the patch itself later):
Please send new versions of patches as roots of new threads, not in
reply to any earlier thread. And please adhere to the To: vs Cc:
separation there, too.
Jan
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