The short description of VIRTIO_BALLOON_F_PAGE_POISON in the feature bits section says:
A hint to the device, that the driver will immediately write poison_val to pages after deflating them.
This is inconsistent with the actual normative requirements in two ways:
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Only mentions deflation: The feature also governs page reporting behavior. The driver MUST initialize reported pages with poison_val before reporting them (Free Page Reporting driver normative), and the device MUST NOT modify reported pages to a value other than poison_val (Free Page Reporting device normative). The short description omits this.
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Calls it a "hint": The driver normative has MUST statements ("The driver MUST initialize the deflated pages with poison_val"), so this is not merely a hint -- it is a binding requirement.
The short description of VIRTIO_BALLOON_F_PAGE_POISON in the feature bits section says:
This is inconsistent with the actual normative requirements in two ways:
Only mentions deflation: The feature also governs page reporting behavior. The driver MUST initialize reported pages with poison_val before reporting them (Free Page Reporting driver normative), and the device MUST NOT modify reported pages to a value other than poison_val (Free Page Reporting device normative). The short description omits this.
Calls it a "hint": The driver normative has MUST statements ("The driver MUST initialize the deflated pages with poison_val"), so this is not merely a hint -- it is a binding requirement.