> -----Original Message-----
> From: Wood Scott-B07421
> Sent: Tuesday, July 29, 2014 11:20 PM
> To: Alexander Graf
> Cc: Bhushan Bharat-R65777; kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; Yoder
> Stuart-B08248
> Subject: Re: [PATCH 6/6] KVM: PPC: BOOKE: Emulate debug registers and exception
>
> On Tue, 2014-07-29 at 16:06 +0200, Alexander Graf wrote:
> > On 29.07.14 00:33, Scott Wood wrote:
> > > On Mon, 2014-07-28 at 16:04 +0200, Alexander Graf wrote:
> > >> On 11.07.14 10:39, Bharat Bhushan wrote:
> > >>> This patch emulates debug registers and debug exception to support
> > >>> guest using debug resource. This enables running gdb/kgdb etc in
> > >>> guest.
> > >>>
> > >>> On BOOKE architecture we cannot share debug resources between QEMU
> > >>> and guest because:
> > >>> When QEMU is using debug resources then debug exception must
> > >>> be always enabled. To achieve this we set MSR_DE and also set
> > >>> MSRP_DEP so guest cannot change MSR_DE.
> > >>>
> > >>> When emulating debug resource for guest we want guest
> > >>> to control MSR_DE (enable/disable debug interrupt on need).
> > >>>
> > >>> So above mentioned two configuration cannot be supported
> > >>> at the same time. So the result is that we cannot share
> > >>> debug resources between QEMU and Guest on BOOKE architecture.
> > >>>
> > >>> In the current design QEMU gets priority over guest, this means
> > >>> that if QEMU is using debug resources then guest cannot use them
> > >>> and if guest is using debug resource then QEMU can overwrite them.
> > >>>
> > >>> Signed-off-by: Bharat Bhushan <Bharat.Bhushan@xxxxxxxxxxxxx>
> > >>> ---
> > >>> Hi Alex,
> > >>>
> > >>> I thought of having some print in register emulation if QEMU is
> > >>> using debug resource, Also when QEMU overwrites guest written
> > >>> values but that looks excessive. If I uses some variable which get
> > >>> set when guest starts using debug registers and check in debug set
> > >>> ioctl then that look ugly. Looking for suggestions
> > >> Whatever you do, have QEMU do the print, not the kernel.
> > > How would that be accomplished? How would the kernel know to exit
> > > to QEMU, and how would the exit reason be conveyed?
> >
> > QEMU is the one forcefully enabling debug and overwriting guest debug
> > registers, so it also knows when it did overwrite valid ones.
>
> QEMU knows when it overwrites the guest values, but it doesn't know if, after
> enabling host debug, the guest tries to write to the debug registers and it gets
> nopped.
Do we want that QEMU first get DBCR0 to know whether it is overwriting whenever set/clear debug register?
> If we keep the EDM setting, then we can at least say the situation is
> no worse than with a JTAG.
Yes
Thanks
-Bharat
>
> -Scott
>
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