Re: [PATCH v7 07/14] KVM: Use gfn instead of hva for mmu_notifier_retry

[Sean Christopherson]

On Mon, Jul 18, 2022, Chao Peng wrote:
> On Fri, Jul 15, 2022 at 01:36:15PM +0200, Gupta, Pankaj wrote:
> > > Currently in mmu_notifier validate path, hva range is recorded and then
> > > checked in the mmu_notifier_retry_hva() from page fault path. However
> > > for the to be introduced private memory, a page fault may not have a hva
> > 
> > As this patch appeared in v7, just wondering did you see an actual bug
> > because of it? And not having corresponding 'hva' occurs only with private
> > memory because its not mapped to host userspace?
> 
> The addressed problem is not new in this version, previous versions I
> also had code to handle it (just in different way). But the problem is:
> mmu_notifier/memfile_notifier may be in the progress of invalidating a
> pfn that obtained earlier in the page fault handler, when happens, we
> should retry the fault. In v6 I used global mmu_notifier_retry() for
> memfile_notifier but that can block unrelated mmu_notifer invalidation
> which has hva range specified.
> 
> Sean gave a comment at https://lkml.org/lkml/2022/6/17/1001 to separate
> memfile_notifier from mmu_notifier but during the implementation I
> realized we actually can reuse the same code for shared and private
> memory if both using gpa range and that can simplify the code handling
> in kvm_zap_gfn_range and some other code (e.g. we don't need two
> versions for memfile_notifier/mmu_notifier).

This should work, though I'm undecided as to whether or not it's a good idea.  
KVM
allows aliasing multiple gfns to a single hva, and so using the gfn could result
in a much larger range being rejected given the simplistic algorithm for 
handling
multiple ranges in kvm_inc_notifier_count().  But I assume such aliasing is 
uncommon,
so I'm not sure it's worth optimizing for.

> Adding gpa range for private memory invalidation also relieves the
> above blocking issue between private memory page fault and mmu_notifier.