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

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Message-ID: <20171129224521.GF3070@tassilo.jf.intel.com>
Date:   Wed, 29 Nov 2017 14:45:21 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org,
        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

On Wed, Nov 29, 2017 at 11:26:30PM +0100, Paolo Bonzini wrote:
> On 29/11/2017 19:20, Andi Kleen wrote:
> > But I haven't looked too closely, but I suspect you'll clobber global
> > kernel debugger state this way.
> 
> I checked all callers of update_debugctlmsr, and couldn't find any that
> could run asynchronously while KVM is caching the value.  For example
> __switch_to_xtra would always run before the sched_in notifier.

True. It would only be a problem if the debugger supported branch stepping
or LBRs, which it doesn't seem to currently.

-Andi