linux-kernel - Re: [PATCH v5 00/10] KVM: x86: Add a cap to disable NX hugepages on a VM

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Date:   Wed, 13 Apr 2022 14:21:32 -0700
From:   David Matlack <dmatlack@...gle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Dunn <daviddunn@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Junaid Shahid <junaids@...gle.com>
Subject: Re: [PATCH v5 00/10] KVM: x86: Add a cap to disable NX hugepages on a VM

On Wed, Apr 13, 2022 at 10:59 AM Ben Gardon <bgardon@...gle.com> wrote:
>
> Given the high cost of NX hugepages in terms of TLB performance, it may
> be desirable to disable the mitigation on a per-VM basis. In the case of public
> cloud providers with many VMs on a single host, some VMs may be more trusted
> than others. In order to maximize performance on critical VMs, while still
> providing some protection to the host from iTLB Multihit, allow the mitigation
> to be selectively disabled.

For the series:

Reviewed-by: David Matlack <dmatlack@...gle.com>

>
> Disabling NX hugepages on a VM is relatively straightforward, but I took this
> as an opportunity to add some NX hugepages test coverage and clean up selftests
> infrastructure a bit.
>
> This series was tested with the new selftest and the rest of the KVM selftests
> on an Intel Haswell machine.
>
> The following tests failed, but I do not believe that has anything to do with
> this series:
>         userspace_io_test
>         vmx_nested_tsc_scaling_test
>         vmx_preemption_timer_test
>
> Changelog:
> v1->v2:
>         Dropped the complicated memslot refactor in favor of Ricardo Koller's
>         patch with a similar effect.
>         Incorporated David Dunn's feedback and reviewed by tag: shortened waits
>         to speed up test.
> v2->v3:
>         Incorporated a suggestion from David on how to build the NX huge pages
>         test.
>         Fixed a build breakage identified by David.
>         Dropped the per-vm nx_huge_pages field in favor of simply checking the
>         global + per-VM disable override.
>         Documented the new capability
>         Separated out the commit to test disabling NX huge pages
>         Removed permission check when checking if the disable NX capability is
>         supported.
>         Added test coverage for the permission check.
> v3->v4:
>         Collected RB's from Jing and David
>         Modified stat collection to reduce a memory allocation [David]
>         Incorporated various improvments to the NX test [David]
>         Changed the NX disable test to run by default [David]
>         Removed some now unnecessary commits
>         Dropped the code to dump KVM stats from the binary stats test, and
>         factor out parts of the existing test to library functions instead.
>         [David, Jing, Sean]
>         Dropped the improvement to a debugging log message as it's no longer
>         relevant to this series.
> v4->v5:
>         Incorporated cleanup suggestions from David and Sean
>         Added a patch with style fixes for the binary stats test from Sean
>         Added a restriction that NX huge pages can only be disabled before
>         vCPUs are created [Sean]
>
> Ben Gardon (9):
>   KVM: selftests: Remove dynamic memory allocation for stats header
>   KVM: selftests: Read binary stats header in lib
>   KVM: selftests: Read binary stats desc in lib
>   KVM: selftests: Read binary stat data in lib
>   KVM: selftests: Add NX huge pages test
>   KVM: x86: Fix errant brace in KVM capability handling
>   KVM: x86/MMU: Allow NX huge pages to be disabled on a per-vm basis
>   KVM: selftests: Factor out calculation of pages needed for a VM
>   KVM: selftests: Test disabling NX hugepages on a VM
>
> Sean Christopherson (1):
>   KVM: selftests: Clean up coding style in binary stats test
>
>  Documentation/virt/kvm/api.rst                |  13 +
>  arch/x86/include/asm/kvm_host.h               |   2 +
>  arch/x86/kvm/mmu.h                            |   9 +-
>  arch/x86/kvm/mmu/spte.c                       |   7 +-
>  arch/x86/kvm/mmu/spte.h                       |   3 +-
>  arch/x86/kvm/mmu/tdp_mmu.c                    |   3 +-
>  arch/x86/kvm/x86.c                            |  25 +-
>  include/uapi/linux/kvm.h                      |   1 +
>  tools/testing/selftests/kvm/Makefile          |  10 +
>  .../selftests/kvm/include/kvm_util_base.h     |  13 +
>  .../selftests/kvm/kvm_binary_stats_test.c     | 142 ++++++-----
>  tools/testing/selftests/kvm/lib/kvm_util.c    | 232 ++++++++++++++++--
>  .../selftests/kvm/x86_64/nx_huge_pages_test.c | 206 ++++++++++++++++
>  .../kvm/x86_64/nx_huge_pages_test.sh          |  25 ++
>  14 files changed, 597 insertions(+), 94 deletions(-)
>  create mode 100644 tools/testing/selftests/kvm/x86_64/nx_huge_pages_test.c
>  create mode 100755 tools/testing/selftests/kvm/x86_64/nx_huge_pages_test.sh
>
> --
> 2.35.1.1178.g4f1659d476-goog
>