Re: PML (Page Modification Logging) design for Xen

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 11/03/15 10:59, George Dunlap wrote:
> On Tue, Feb 17, 2015 at 10:19 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>> On 12.02.15 at 03:39, <kai.huang@linux.intel.com> wrote:
>>> On 02/11/2015 07:52 PM, Andrew Cooper wrote:
>>>> On 11/02/15 08:28, Kai Huang wrote:
>>>>> Design
>>>>> ======
>>>>>
>>>>> - PML feature is used globally
>>>>>
>>>>> A new Xen boot parameter, say 'opt_enable_pml', will be introduced to
>>>>> control PML feature detection, and PML feature will only be detected
>>>>> if opt_enable_pml = 1. Once PML feature is detected, it will be used
>>>>> for dirty logging for all domains globally. Currently we don't support
>>>>> to use PML on basis of per-domain as it will require additional
>>>>> control from XL tool.
>>>> Rather than adding in a new top level command line option for an ept
>>>> subfeature, it would be preferable to add an "ept=" option which has
>>>> "pml" as a sub boolean.
>>> Which is good to me, if Jan agrees.
>>>
>>> Jan, which do you prefer here?
>> A single "ept=" option as Andrew suggested.
> Sorry to be coming late to this party -- what's the logic behind
> having this enabled with "ept="? You're not changing anything about
> how EPT itself works; you're adding a secondary feature which happens
> to depend on ept.  Is there another hypervisor command-line option you
> had in mind that works this way?

iommu=

>
> It might also be nice to be able to enable or disable this feature
> with a sysctl call; but that's just a nice-to-have.

This feature should either be used or not.  It is impractical to
enable/disable at runtime.

However, it absolutely wants a knob for tweaking.  The likely
consequence of a bug in the implementation is VM memory corruption on
migrate, and you can get away with missing a large amount of a domains
memory before it blows up noticeably.

~Andrew