Re: [Patch 2/4] KVM:SVM: Introduce set_spte_notify support

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Wed, Aug 19, 2020 at 11:03:48AM -0500, Eric van Tassell wrote:
> 
> 
> On 8/3/20 11:27 AM, Sean Christopherson wrote:
> >On Sun, Aug 02, 2020 at 03:53:54PM -0500, Eric van Tassell wrote:
> >>
> >>On 7/31/20 3:25 PM, Sean Christopherson wrote:
> >>>On Fri, Jul 24, 2020 at 06:54:46PM -0500, eric van tassell wrote:
> >>>>Improve SEV guest startup time from O(n) to a constant by deferring
> >>>>guest page pinning until the pages are used to satisfy nested page faults.
> >>>>
> >>>>Implement the code to do the pinning (sev_get_page) and the notifier
> >>>>sev_set_spte_notify().
> >>>>
> >>>>Track the pinned pages with xarray so they can be released during guest
> >>>>termination.
> >>>
> >>>I like that SEV is trying to be a better citizen, but this is trading one
> >>>hack for another.
> >>>
> >>>   - KVM goes through a lot of effort to ensure page faults don't need to
> >>>     allocate memory, and this throws all that effort out the window.
> >>>
> >>can you elaborate on that?
> >
> >mmu_topup_memory_caches() is called from the page fault handlers before
> >acquiring mmu_lock to pre-allocate shadow pages, PTE list descriptors, GFN
> >arrays, etc... that may be needed to handle the page fault.  This allows
> >using standard GFP flags for the allocation and obviates the need for error
> >handling in the consumers.
> >
> 
> I see what you meant. The issue that causes us to use this approach is that
> we need to be able to unpin the pages when the VM exits.

Yes, but using a software available flag in the SPTE to track pinned pages
should be very doable.