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

[Chao Peng]

On Mon, Jul 18, 2022 at 03:26:34PM +0000, Sean Christopherson wrote:
> 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.

That can be a real problem for current v7 code, __kvm_handle_hva_range()
loops all possible gfn_range for a given hva_range but the
on_lock/on_unlock is invoked only once, this should work for hva_range,
but not gfn_range since we can have multiple of them.

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