Re: [PATCH v7.1] KVM: SVM: Add Page modification logging support

["Nikunj A. Dadhania" <nikunj@amd.com>]

On 5/29/2026 12:08 PM, Nikunj A Dadhania wrote:
> Currently, dirty logging relies on write protecting guest memory and
> marking dirty GFNs during subsequent write faults. This method works but
> incurs overhead due to additional write faults for each dirty GFN.
> 
> Implement support for the Page Modification Logging (PML) feature, a
> hardware-assisted method for efficient dirty logging. PML automatically
> logs dirty GPA[51:12] to a 4K buffer when the CPU sets NPT D-bits. Two new
> VMCB fields are utilized: PML_ADDR and PML_INDEX. The PML_INDEX is
> initialized to 511 (8 bytes per GPA entry), and the CPU decreases the
> PML_INDEX after logging each GPA. When the PML buffer is full, a
> VMEXIT(PML_FULL) with exit code 0x407 is generated.
> 
> Since PML_INDEX in the VMCB control area remains valid after an intercepted
> SHUTDOWN, only initialize it on reset and leave it unchanged on INIT to
> avoid discarding already-logged entries that haven't been flushed.
> 
> PML operates on guest physical addresses at the NPT level, tracking D-bit
> updates in page tables rather than memory content. This allows it to work
> identically for normal and confidential computing guests
> (SEV/SEV-ES/SEV-SNP), enabling cpu_dirty_log_size to be set uniformly for
> all AMD VMs without special-casing encrypted guests.
> 
> Use vmcb01 directly when updating PML controls to ensure L1's state
> remains correct, as svm->vmcb points to vmcb02 when L2 is active.
> 
> PML is not enabled in hardware for nested guests; treat PML_FULL as
> unexpected exits.
> 
> Add a new module parameter to enable/disable PML, and enable it by default
> when supported.
> 
> Acked-by: Kai Huang <kai.huang@intel.com>
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
> ---
> 
> Sending updated 7/7 patch here, rather than reposting the full series.
> 
> Sean, any remaining comments? Happy to send v8 if needed.

A gentle reminder

> v7 -> v7.1:
> * Drop redundant pml_addr=0 in nested_vmcb02_prepare_control() (Yosry)
> * Fix misleading comment, pml_index=-1 is hardening not stale-data cleanup (Yosry, Sean)
> * Route nested SVM_EXIT_PML_FULL to KVM and treat it as unexpected exit (Sean)

Regards,
Nikunj