linux-kernel - Re: [PATCH v2] KVM: Expose the split lock detection feature to guest VM

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Message-ID: <7813a04b-8058-538b-8c9b-cdd01ce119ac@redhat.com>
Date:   Wed, 4 Jul 2018 15:36:40 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jingqi Liu <jingqi.liu@...el.com>
Cc:     rkrcmar@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, wei.w.wang@...el.com,
        Robert Hoo <robert.hu@...ux.intel.com>
Subject: Re: [PATCH v2] KVM: Expose the split lock detection feature to guest
 VM

On 04/07/2018 15:06, Jingqi Liu wrote:
> A new control bit(bit 29) in the TEST_CTRL MSR will be introduced
> to enable detection of split locks.
> 
> When bit 29 of the TEST_CTRL(33H) MSR is set, the processor
> causes an #AC exception to be issued instead of suppressing LOCK on
> bus(during split lock access). A previous control bit (bit 31)
> in this MSR causes the processor to disable LOCK# assertion for
> split locked accesses when set. When bits 29 and 31 are both set,
> bit 29 takes precedence.
> 
> The release document ref below link:
> https://software.intel.com/sites/default/files/managed/c5/15/\
> architecture-instruction-set-extensions-programming-reference.pdf
> This patch has a dependency on https://lkml.org/lkml/2018/5/27/78.
> 
> Signed-off-by: Jingqi Liu <jingqi.liu@...el.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  1 +
>  arch/x86/kvm/vmx.c              | 77 +++++++++++++++++++++++++++++++++++++++++
>  arch/x86/kvm/x86.c              | 10 ++++++
>  arch/x86/kvm/x86.h              |  5 +++
>  include/uapi/linux/kvm.h        |  1 +
>  5 files changed, 94 insertions(+)

Checking for split lock is done with the MSR_TEST_CTL too, so you should
not use a capability to expose the availability to KVM userspace.
Instead you should expose the contents of MSR_TEST_CTL on the host, in a
similar way to https://marc.info/?l=kvm&m=152998661713547&w=2.

Please coordinate with Robert Hu on the QEMU patches too, because he's
working on the infrastructure to use KVM_GET_MSR_FEATURE_INDEX_LIST in QEMU.

Thanks,

Paolo