Re: [PATCH v5 7/8] KVM: SVM: Add Page modification logging support

["Huang, Kai" <kai.huang@intel.com>]

On Mon, 2026-01-05 at 06:36 +0000, 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.
> 
> Disable PML for nested guests.
> 
> PML is enabled by default when supported and can be disabled via the 'pml'
> module parameter.

Nit:

If a new version is needed, use imperative mode:

  Add a new module parameter to enable/disable PML, and enable it by 
  default when supported.

> 
> Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>

It's a bit weird for me to review, but I did anyway and it seems fine to
me, so:

Acked-by: Kai Huang <kai.huang@intel.com>

One minor thing below ...

[...]

> @@ -748,12 +748,19 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
>  						V_NMI_BLOCKING_MASK);
>  	}
>  
> -	/* Copied from vmcb01.  msrpm_base can be overwritten later.  */
> +	/* Copied from vmcb01. msrpm_base/nested_ctl can be overwritten later. */
>  	vmcb02->control.nested_ctl = vmcb01->control.nested_ctl;
>  	vmcb02->control.iopm_base_pa = vmcb01->control.iopm_base_pa;
>  	vmcb02->control.msrpm_base_pa = vmcb01->control.msrpm_base_pa;
>  	vmcb_mark_dirty(vmcb02, VMCB_PERM_MAP);
>  
> +	/* Disable PML for nested guest as the A/D update is emulated by MMU */

This comment isn't accurate to me.  I think the key reason is, for L2 if
PML enabled the recorded GPA will be L2's GPA, but not L1's.

Please update the comment if a new version is needed?

> +	if (pml) {
> +		vmcb02->control.nested_ctl &= ~SVM_NESTED_CTL_PML_ENABLE;
> +		vmcb02->control.pml_addr = 0;
> +		vmcb02->control.pml_index = -1;
> +	}
> +
>