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Date:   Wed, 29 Nov 2017 11:05:46 -0800
From:   Jim Mattson <>
To:     Andi Kleen <>
Cc:     Wanpeng Li <>,
        LKML <>,
        kvm list <>,
        Paolo Bonzini <>,
        Radim Krčmář <>,
        Wanpeng Li <>
Subject: Re: [PATCH] KVM: VMX: Cache IA32_DEBUGCTL in memory

An alternative is to give the L1 guest read permission for this MSR in
the MSR permission bitmaps. It's still going to be ~80 cycles, but
that's better than the cost of a VM-exit/VM-entry round-trip.

On Wed, Nov 29, 2017 at 10:20 AM, Andi Kleen <> wrote:
> Wanpeng Li <> writes:
>> From: Wanpeng Li <>
>> MSR_IA32_DEBUGCTLMSR is zeroed on VMEXIT, so it is saved/restored
>> each time during world switch. Jim from Google pointed out that
>> when running schbench in L2, vmx_vcpu_run will occupy 4% cpu time,
>> and the 25% of vmx_vcpu_run cpu time is occupied by get_debugctlmsr().
>> This patch caches the host IA32_DEBUGCTL MSR and saves/restores
>> the host IA32_DEBUGCTL msr when guest/host switches to avoid to
>> save/restore each time during world switch.
> FWIW i've seen this too on L2 profiles.
> But I haven't looked too closely, but I suspect you'll clobber global
> kernel debugger state this way.
> You would at least need some interface for KDB etc. to invalidate
> your cache.
> -Andi

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