Messages in this thread | | | Date | Tue, 7 Oct 2008 22:15:35 +0530 | From | "K.Prasad" <> | Subject | Re: [RFC Patch 0/9] Hardware Breakpoint interfaces |
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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!
Thanks, K.Prasad
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