linux-kernel - Re: [PATCH] KVM: VMX: Cache IA32

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Date:   Wed, 29 Nov 2017 10:20:51 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH] KVM: VMX: Cache IA32_DEBUGCTL in memory

Wanpeng Li <kernellwp@...il.com> writes:

> From: Wanpeng Li <wanpeng.li@...mail.com>
>
> 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