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Re: [PATCH v4 9/9] x86/cpuid: Advertise XEN_HVM_CPUID_EXT_DEST_ID when device model opts in
- To: Julian Vetter <julian.vetter@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 19 Aug 2026 17:01:06 +0200
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- Cc: Anthony PERARD <anthony.perard@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 19 Aug 2026 15:01:16 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 27.04.2026 15:54, Julian Vetter wrote:
> Set the XEN_HVM_CPUID_EXT_DEST_ID bit in the HVM hypervisor CPUID leaf
> based on the domain-level ext_dest_id flag, which is locked at domain
> creation time by taking the AND across all registered ioreq servers that
> set XEN_DMOP_IOREQ_SERVER_EXT_DEST_ID. This guarantees that the bit is
> only advertised when every active device model will use
> XEN_DMOP_bind_pt_msi_irq for passthrough MSIs, so Xen can decode the
> extended destination bits from the raw MSI address internally.
>
> After creation_finished the locked d->arch.hvm.ext_dest_id is used
> directly, providing a stable and migration-safe value independent of
> whether ioreq servers have been re-registered yet. Before
> creation_finished the dynamic per-server check is used so toolstack
> queries during domain setup reflect the current state.
What toolstack queries to you have in mind here? Did you check ...
> --- a/xen/arch/x86/cpuid.c
> +++ b/xen/arch/x86/cpuid.c
> @@ -1,3 +1,4 @@
> +#include <xen/ioreq.h>
> #include <xen/sched.h>
> #include <xen/types.h>
> #include <xen/version.h>
> @@ -148,6 +149,18 @@ static void cpuid_hypervisor_leaves(const struct vcpu
> *v, uint32_t leaf,
... call sites of this function? It's solely guest_cpuid(), and all
callers of the latter act on current. I.e. if there was a need for a
toolstack to find out (transient) state, it would need to be through
a different interface anyway.
Jan
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