Re: [PATCH v2] KVM: Move gfn_to_pfn_cache invalidation to invalidate_range_end hook

[Sean Christopherson <seanjc@google.com>]

On Tue, Aug 20, 2024, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
> 
> The existing retry loop in hva_to_pfn_retry() is extremely pessimistic.
> If there are any concurrent invalidations running, it's effectively just
> a complex busy wait loop because its local mmu_notifier_retry_cache()
> function will always return true.
> 
> Since multiple invalidations can be running in parallel, this can result
> in a situation where hva_to_pfn_retry() just backs off and keep retrying
> for ever, not making any progress.
> 
> Solve this by being a bit more selective about when to retry.
> 
> Introduce a separate 'needs invalidation' flag to the GPC, which allows
> it to be marked invalid even while hva_to_pfn_retry() has dropped the
> lock to map the newly-found PFN. This allows the invalidation to moved
> to the range_end hook, and the retry loop only occurs for a given GPC if
> its particular uHVA is affected.
> 
> However, the contract for invalidate_range_{start,end} is not like a
> simple TLB; the pages may have been freed by the time the end hook is
> called. A "device" may not create new mappings after the _start_ hook is
> called. To meet this requirement, hva_to_pfn_retry() now waits until no
> invalidations are currently running which may affect its uHVA, before
> finally setting the ->valid flag and returning.

Please split this into 3 patches:

 1. Add range-based GPC retry
 2. Add the wait mechanism.
 3. Add the needs_invalidation logic.

#1 and #2 make sense to me, but I'm struggling to understanding why #3 is needed.
KVM absolutely must not touch the memory after .invalidate_range_start(), so I
don't see what is gained by deferring invalidation to invalidate_range_end().