linux-kernel - Re: [PATCH v5 0/7] Lazily allocate memslot rmaps

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Message-ID: <85bdd768-d7a3-100c-aade-ef5910b6e729@redhat.com>
Date:   Mon, 24 May 2021 14:43:56 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Kai Huang <kai.huang@...el.com>,
        Keqian Zhu <zhukeqian1@...wei.com>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH v5 0/7] Lazily allocate memslot rmaps

On 18/05/21 19:34, Ben Gardon wrote:
> This series enables KVM to save memory when using the TDP MMU by waiting
> to allocate memslot rmaps until they are needed. To do this, KVM tracks
> whether or not a shadow root has been allocated. In order to get away
> with not allocating the rmaps, KVM must also be sure to skip operations
> which iterate over the rmaps. If the TDP MMU is in use and we have not
> allocated a shadow root, these operations would essentially be op-ops
> anyway. Skipping the rmap operations has a secondary benefit of avoiding
> acquiring the MMU lock in write mode in many cases, substantially
> reducing MMU lock contention.
> 
> This series was tested on an Intel Skylake machine. With the TDP MMU off
> and on, this introduced no new failures on kvm-unit-tests or KVM selftests.
> 
> Changelog:
> v2:
> 	Incorporated feedback from Paolo and Sean
> 	Replaced the memslot_assignment_lock with slots_arch_lock, which
> 	has a larger critical section.
> 
> v3:
> 	Removed shadow_mmu_active as suggested by Sean
> 	Removed everything except adding a return value to
> 	kvm_mmu_init_tdp_mmu from patch 1 of v2
> 	Added RCU protection and better memory ordering for installing the
> 	memslot rmaps as suggested by Paolo
> 	Reordered most of the patches
> 
> v4:
> 	Renamed functions to allocate and free memslots based on feedback
> 	from David.
> 	Eliminated the goto in memslot_rmap_alloc, as David suggested.
> 	Eliminated kvm_memslots_have_rmaps and updated comments on uses of
> 	memslots_have_rmaps. Suggested by Paolo.
> 	Changed the description on patch 7 to one Paolo suggested.
> 	Collected Reviewed-by tags from David.
> 	Dropped the patch to add RCU notations to rmap accesses.
> 
> v5:
> 	Responding to comments from Sean.
> 	Improved comments
> 	Swapped args to kvm_copy_memslots to match memcpy
> 	Fixed some line wrap and declaration style issues
> 	No longer check if memslots have rmaps before operations which
> 	iterate through active_mmu_pages
> 	Re-added the kvm_memslots_have_rmaps helper
> 	Fixed a couple missing unlocks for the slots_arch_lock

Queued (with minor conflicts), thanks!

Paolo