linux-kernel - Re: [PATCH v2 0/4] KVM: x86/mmu: Zapping and recycling cleanups

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Message-ID: <59e690be-b0a6-4beb-fd7c-633186b96ece@redhat.com>
Date:   Fri, 3 Jul 2020 19:18:49 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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
Subject: Re: [PATCH v2 0/4] KVM: x86/mmu: Zapping and recycling cleanups

On 23/06/20 21:35, Sean Christopherson wrote:
> Semi-random, but related, changes that deal with the handling of active
> root shadow pages during zapping and the zapping of arbitary/old pages.
> 
> Patch 1 changes the low level handling to keep zapped active roots off the
> active page list.  KVM already relies on the vCPU to explicitly free the
> root, putting invalid root pages back on the list is just a quirk of the
> implementation.
> 
> Patches 2 reworks the MMU page recycling to batch zap pages instead of
> zapping them one at a time.  This provides better handling for active root
> pages and also avoids multiple remote TLB flushes.
> 
> Patch 3 applies the batch zapping to the .shrink_scan() path.  This is a
> significant change in behavior, i.e. is the scariest of the changes, but
> unless I'm missing something it provides the intended functionality that
> has been lacking since shrinker support was first added.
> 
> Patch 4 changes the page fault handlers to return an error to userspace
> instead of restarting the guest if there are no MMU pages available.  This
> is dependent on patch 2 as theoretically the old recycling flow could
> prematurely bail if it encountered an active root.
> 
> v2:
>   - Add a comment for the list shenanigans in patch 1. [Paolo]
>   - Add patches 2-4.
>   - Rebased to kvm/queue, commit a037ff353ba6 ("Merge branch ...")
> 
> Sean Christopherson (4):
>   KVM: x86/mmu: Don't put invalid SPs back on the list of active pages
>   KVM: x86/mmu: Batch zap MMU pages when recycling oldest pages
>   KVM: x86/mmu: Batch zap MMU pages when shrinking the slab
>   KVM: x86/mmu: Exit to userspace on make_mmu_pages_available() error
> 
>  arch/x86/kvm/mmu/mmu.c         | 94 +++++++++++++++++++++-------------
>  arch/x86/kvm/mmu/paging_tmpl.h |  3 +-
>  2 files changed, 61 insertions(+), 36 deletions(-)
> 

Queued, thanks.

Paolo