Re: [PATCH] KVM: Use thread debug register storage instead of kvm specific data

[Jan Kiszka <jan.kiszka@siemens.com>]

Jan Kiszka wrote:
> Marcelo Tosatti wrote:
>> On Tue, Sep 01, 2009 at 12:44:08PM +0300, Avi Kivity wrote:
>>> Instead of saving the debug registers from the processor to a kvm data
>>> structure, rely in the debug registers stored in the thread structure.
>>> This allows us not to save dr6 and dr7.
>>>
>>> Reduces lightweight vmexit cost by 350 cycles, or 11 percent.
>> Is this kgdb safe?
> 
> Nope, kgdb writes directly to the debug registers.
> 
> I vaguely recall someone trying to push a debug register management
> framework. Did it hit mainline in the meantime? I do not find any trace
> on quick glance, at least not in kgdb.

For the time being, falling back to conservative save/restore if kgdb is
configured in and kgdb_connected != 0 should allow us to apply this
optimization otherwise.

Jan

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