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SubjectRe: [RFC Patch 0/9] Hardware Breakpoint interfaces
On Tue, Oct 07, 2008 at 04:36:54PM +0200, Avi Kivity wrote:
> K.Prasad wrote:
>> On Tue, Oct 07, 2008 at 02:29:05PM +0200, Avi Kivity wrote:
>>> K.Prasad wrote:
>>>> - Enable KGDB and KVM to use the register_kernel_hw_breakpoint()
>>>> interface for their HW Breakpoint usage, in the absence of which
>>>> they will be broken during simultaneous use.
>>> KVM conceptually isn't a kernel use of the debug registers. KVM
>>> modifies the debug registers while the guest is running, and restores
>>> them after the guest returns.
>>> Right now, as an optimization, KVM defers restoring the debug
>>> registers until after the next context switch out of the kvm task,
>>> or until the next exit to userspace (whichever comes earlier); we
>>> should change this to avoid the deferral if kernel breakpoints are
>>> in effect. This will allow simultaneous use of KVM breakpoints and
>>> kernel breakpoints.
>> The patch posted provides interface for using HW breakpoints on both
>> user- and kernel-space breakpoints. KVM's user-space address breakpoint
>> requirements, after code-modification, should be made to use
>> register_user_hw_breakpoint() interface (presently un-exported but will
>> be changed subsequently) to help maintain a system-wide consistent view on the
>> availability of HW Breakpoint registers.
> Correcting myself, actually kvm breakpoints are in a third namespace.
> You could have kernel breakpoints, user breakpoints, and guest
> breakpoints coexisting.
>> Presently I find plenty of set_debugreg() calls from kvm/ which will
>> modify the registers directly and break the breakpoint register
>> management brought-in through the patch.
> If kvm restores the registers, should there be any problem?
No, it should be fine although I don't understand how the exception
handler is invoked in KVM without the use of notifier or a hook in
die_debug (or have they replaced code at a layer much below that?).

Apart from the doubt I've stated above, if they operate by replacing
the breakpoint register contents before a context switch from KVM to
other processes, they might in fact help maximise its utilisation.

Thanks for the clarification!


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