linux-kernel - Re: [PATCH 00/14] KVM: x86/mmu: Dirty logging fixes and improvements

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Date:   Thu, 18 Feb 2021 13:57:43 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
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, Ben Gardon <bgardon@...gle.com>,
        Makarand Sonare <makarandsonare@...gle.com>
Subject: Re: [PATCH 00/14] KVM: x86/mmu: Dirty logging fixes and improvements

On 13/02/21 01:50, Sean Christopherson wrote:
> Paolo, this is more or less ready, but on final read-through before
> sending I realized it would be a good idea to WARN during VM destruction
> if cpu_dirty_logging_count is non-zero.  I wanted to get you this before
> the 5.12 window opens in case you want the TDP MMU fixes for 5.12.  I'll
> do the above change and retest next week (note, Monday is a US holiday).
> 
> On to the code...
> 
> This started out as a small tweak to collapsible SPTE zapping in the TDP
> MMU, and ended up as a rather large overhaul of CPU dirty logging, a.k.a.
> PML.
> 
> Four main highlights:
> 
>    - Do a more precise check on whether or not a SPTE should be zapped to
>      rebuild it as a large page.
>    - Disable PML when running L2.  PML is fully emulated for L1 VMMs, thus
>      enabling PML in L2 can only hurt and never help.
>    - Drop the existing PML kvm_x86_ops.  They're basically trampolines into
>      the MMU, and IMO do far more harm than good.
>    - Turn on PML only when it's needed instead of setting all dirty bits to
>      soft disable PML.
> 
> What led me down the rabbit's hole of ripping out the existing PML
> kvm_x86_ops isn't really shown here.  Prior to incorporating Makarand's
> patch, which allowed for the wholesale remove of setting dirty bits,
> I spent a bunch of time poking around the "set dirty bits" code.  My
> original changes optimized that path to skip setting dirty bits in the
> nested MMU, since the nested MMU relies on write-protection and not PML.
> That in turn allowed the TDP MMU zapping to completely skip walking the
> rmaps, but doing so based on a bunch of callbacks was a twisted mess.
> 
> Happily, those patches got dropped in favor of nuking the code entirely.
> 
> Ran selftest and unit tests, and migrated actual VMs on AMD and Intel,
> with and without TDP MMU, and with and without EPT.  The AMD system I'm
> testing on infinite loops on the reset vector due to a #PF when NPT is
> disabled, so that didn't get tested.  That reproduces with kvm/next,
> I'll dig into it next week (no idea if it's a KVM or hardware issue).
> 
> For actual migration, I ran kvm-unit-tests in L1 along with stress to
> hammer memory, and verified migration was effectively blocked until the
> stress threads were killed (I didn't feel like figuring out how to
> throttle the VM).
> 
> Makarand Sonare (1):
>    KVM: VMX: Dynamically enable/disable PML based on memslot dirty
>      logging
> 
> Sean Christopherson (13):
>    KVM: x86/mmu: Expand collapsible SPTE zap for TDP MMU to ZONE_DEVICE
>      pages
>    KVM: x86/mmu: Don't unnecessarily write-protect small pages in TDP MMU
>    KVM: x86/mmu: Split out max mapping level calculation to helper
>    KVM: x86/mmu: Pass the memslot to the rmap callbacks
>    KVM: x86/mmu: Consult max mapping level when zapping collapsible SPTEs
>    KVM: nVMX: Disable PML in hardware when running L2
>    KVM: x86/mmu: Expand on the comment in
>      kvm_vcpu_ad_need_write_protect()
>    KVM: x86/mmu: Make dirty log size hook (PML) a value, not a function
>    KVM: x86: Move MMU's PML logic to common code
>    KVM: x86: Further clarify the logic and comments for toggling log
>      dirty
>    KVM: x86/mmu: Don't set dirty bits when disabling dirty logging w/ PML
>    KVM: x86: Fold "write-protect large" use case into generic
>      write-protect
>    KVM: x86/mmu: Remove a variety of unnecessary exports
> 
>   arch/x86/include/asm/kvm-x86-ops.h |   6 +-
>   arch/x86/include/asm/kvm_host.h    |  36 +----
>   arch/x86/kvm/mmu/mmu.c             | 203 +++++++++--------------------
>   arch/x86/kvm/mmu/mmu_internal.h    |   7 +-
>   arch/x86/kvm/mmu/tdp_mmu.c         |  66 +---------
>   arch/x86/kvm/mmu/tdp_mmu.h         |   3 +-
>   arch/x86/kvm/vmx/nested.c          |  34 +++--
>   arch/x86/kvm/vmx/vmx.c             |  94 +++++--------
>   arch/x86/kvm/vmx/vmx.h             |   2 +
>   arch/x86/kvm/x86.c                 | 145 +++++++++++++--------
>   10 files changed, 230 insertions(+), 366 deletions(-)
> 

Queued 1-9 and 14 until you clarify my doubt about patch 10, thanks.

Paolo