Re: [PATCH v2 2/3] KVM: x86: fix nested guest live migration with PML

[Junaid Shahid <junaids@google.com>]

On 9/27/19 4:15 AM, Paolo Bonzini wrote:
> Shadow paging is fundamentally incompatible with the page-modification
> log, because the GPAs in the log come from the wrong memory map.
> In particular, for the EPT page-modification log, the GPAs in the log come
> from L2 rather than L1.  (If there was a non-EPT page-modification log,
> we couldn't use it for shadow paging because it would log GVAs rather
> than GPAs).
> 
> Therefore, we need to rely on write protection to record dirty pages.
> This has the side effect of bypassing PML, since writes now result in an
> EPT violation vmexit.
> 
> This is relatively easy to add to KVM, because pretty much the only place
> that needs changing is spte_clear_dirty.  The first access to the page
> already goes through the page fault path and records the correct GPA;
> it's only subsequent accesses that are wrong.  Therefore, we can equip
> set_spte (where the first access happens) to record that the SPTE will
> have to be write protected, and then spte_clear_dirty will use this
> information to do the right thing.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

Reviewed-by: Junaid Shahid <junaids@google.com>