linux-kernel - Re: [PATCH] KVM: x86/mmu: Complete prefetch for trailing SPTEs for direct, legacy MMU

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Message-ID: <YR3a21l6TC/gmw3/@google.com>
Date:   Thu, 19 Aug 2021 13:15:23 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <senozhatsky@...gle.com>,
        Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH] KVM: x86/mmu: Complete prefetch for trailing SPTEs for
 direct, legacy MMU

[..]

> Make a final call to direct_pte_prefetch_many() if there are "trailing"
> SPTEs to prefetch, i.e. SPTEs for GFNs following the faulting GFN.  The
> call to direct_pte_prefetch_many() in the loop only handles the case
> where there are !PRESENT SPTEs preceding a PRESENT SPTE.
> 
> E.g. if the faulting GFN is a multiple of 8 (the prefetch size) and all
> SPTEs for the following GFNs are !PRESENT, the loop will terminate with
> "start = sptep+1" and not prefetch any SPTEs.
> 
> Prefetching trailing SPTEs as intended can drastically reduce the number
> of guest page faults, e.g. accessing the first byte of every 4kb page in
> a 6gb chunk of virtual memory, in a VM with 8gb of preallocated memory,
> the number of pf_fixed events observed in L0 drops from ~1.75M to <0.27M.
> 
> Note, this only affects memory that is backed by 4kb pages as KVM doesn't
> prefetch when installing hugepages.  Shadow paging prefetching is not
> affected as it does not batch the prefetches due to the need to process
> the corresponding guest PTE.  The TDP MMU is not affected because it
> doesn't have prefetching, yet...


Tested-by: Sergey Senozhatsky <senozhatsky@...omium.org>



I ran some tests.


- VM Boot up

From

EPT_VIOLATION    1192184    75.18%     4.40%      0.77us  18020.01us      4.32us ( +-   1.71% )

to

EPT_VIOLATION     947460    69.92%     4.64%      0.69us  34902.15us      5.06us ( +-   1.64% )



- Running test app (in VM)

From

EPT_VIOLATION    6550167    71.05%    11.76%      0.77us  32562.18us      3.51us ( +-   0.36% )

to

EPT_VIOLATION    5489904    68.32%    11.29%      0.71us  16564.19us      3.92us ( +-   0.29% )