linux-kernel - Re: [PATCH v2 0/8] KVM: x86/mmu: ITLB multi-hit workaround fixes

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Message-ID: <4eec29f9-def0-3292-4a88-1f6ff3e06b5e@redhat.com>
Date:   Fri, 25 Sep 2020 23:27:15 +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, Junaid Shahid <junaids@...gle.com>
Subject: Re: [PATCH v2 0/8] KVM: x86/mmu: ITLB multi-hit workaround fixes

On 23/09/20 20:37, Sean Christopherson wrote:
> Patch 1 is a minor fix for a very theoretical bug where KVM could skip
> the final "commit zap" when recovering shadow pages for the NX huge
> page mitigation.
> 
> Patch 2 is cleanup that's made possible by patch 1.
> 
> Patches 3-5 are the main course and fix bugs in the NX huge page
> accounting where shadow pages are incorrectly added to the list of
> disallowed huge pages.  KVM doesn't actually check to see if the page
> could actually have been a large page when adding to the disallowed list.
> This result in what are effectively spurious zaps.  The biggest issue is
> likely with shadow pages in the upper levels, i.e. levels 3 and 4, as they
> are either unlikely to be huge (1gb) or flat out can't be huge (512tb).
> And because of the way KVM zaps, the upper levels will be zapped first,
> i.e. KVM is likely zapping and rebuilding a decent number of its shadow
> pages for zero benefit.
> 
> Ideally, patches 3-5 would be a single patch to ease backporting.  In the
> end, I decided the change is probably not suitable for stable as at worst
> it creates an infrequent performance spike (assuming the admin isn't going
> crazy with the recovery frequency), and it's far from straightforward or
> risk free.  Cramming everything into a single patch was a mess.
> 
> Patches 6-8 are cleanups in related code.  The 'hlevel' name in particular
> has been on my todo list for a while.
> 
> v2:
>   - Rebased to kvm/queue, commit e1ba1a15af73 ("KVM: SVM: Enable INVPCID
>     feature on AMD").
> 
> Sean Christopherson (8):
>   KVM: x86/mmu: Commit zap of remaining invalid pages when recovering
>     lpages
>   KVM: x86/mmu: Refactor the zap loop for recovering NX lpages
>   KVM: x86/mmu: Move "huge page disallowed" calculation into mapping
>     helpers
>   KVM: x86/mmu: Capture requested page level before NX huge page
>     workaround
>   KVM: x86/mmu: Account NX huge page disallowed iff huge page was
>     requested
>   KVM: x86/mmu: Rename 'hlevel' to 'level' in FNAME(fetch)
>   KVM: x86/mmu: Hoist ITLB multi-hit workaround check up a level
>   KVM: x86/mmu: Track write/user faults using bools
> 
>  arch/x86/kvm/mmu/mmu.c         | 58 +++++++++++++++++++++-------------
>  arch/x86/kvm/mmu/paging_tmpl.h | 39 ++++++++++++-----------
>  2 files changed, 57 insertions(+), 40 deletions(-)
> 

Queued, thanks.

Paolo