x86/vmx: fix posted interrupts usage of msi_desc->msg field

The current usage of msi_desc->msg in vmx_pi_update_irte() will make the
field contain a translated MSI message, instead of the expected
untranslated one.  This breaks dump_msi(), that use the data in
msi_desc->msg to print the interrupt details.

Fix this by introducing a dummy local msi_msg, and use it with
iommu_update_ire_from_msi().  vmx_pi_update_irte() relies on the MSI
message not changing, so there's no need to propagate the resulting msi_msg
to the hardware, and the contents can be ignored.

Additionally add a comment to clarify that msi_desc->msg must always
contain the untranslated MSI message.

Fixes: a5e25908d1 ('VT-d: introduce new fields in msi_desc to track binding with guest interrupt')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: 30f0e55a79
master date: 2025-03-12 13:32:30 +0100

xen/arch/x86/hvm/vmx/vmx.c

@@ -397,6 +397,7 @@ static int cf_check vmx_pi_update_irte(const struct vcpu *v,
     const struct pi_desc *pi_desc = v ? &v->arch.hvm.vmx.pi_desc : NULL;
     struct irq_desc *desc;
     struct msi_desc *msi_desc;
+    struct msi_msg msg;
     int rc;
 
     desc = pirq_spin_lock_irq_desc(pirq, NULL);
@@ -411,12 +412,13 @@ static int cf_check vmx_pi_update_irte(const struct vcpu *v,
     }
     msi_desc->pi_desc = pi_desc;
     msi_desc->gvec = gvec;
+    msg = msi_desc->msg;
 
     spin_unlock_irq(&desc->lock);
 
     ASSERT(pcidevs_locked());
 
-    return iommu_update_ire_from_msi(msi_desc, &msi_desc->msg);
+    return iommu_update_ire_from_msi(msi_desc, &msg);
 
  unlock_out:
     spin_unlock_irq(&desc->lock);

xen/arch/x86/include/asm/msi.h

@@ -123,7 +123,7 @@ struct msi_desc {
     int irq;
     int remap_index;         /* index in interrupt remapping table */
 
-    struct msi_msg msg;      /* Last set MSI message */
+    struct msi_msg msg;      /* Last set MSI message (untranslated) */
 };
 
 /*