Re: [PATCH 2/7] powerpc/watchpoint/kvm: Add infrastructure to support 2nd DAWR

[Ravi Bangoria]

Hi Paul,

On 9/2/20 7:31 AM, Paul Mackerras wrote:
> On Thu, Jul 23, 2020 at 03:50:53PM +0530, Ravi Bangoria wrote:
> > kvm code assumes single DAWR everywhere. Add code to support 2nd DAWR.
> > DAWR is a hypervisor resource and thus H_SET_MODE hcall is used to set/
> > unset it. Introduce new case H_SET_MODE_RESOURCE_SET_DAWR1 for 2nd DAWR.
>
> Is this the same interface as will be defined in PAPR and available
> under PowerVM, or is it a new/different interface for KVM?

Yes, kvm hcall interface for 2nd DAWR is same as PowerVM, as defined in PAPR.

> > Also, kvm will support 2nd DAWR only if CPU_FTR_DAWR1 is set.
>
> In general QEMU wants to be able to control all aspects of the virtual
> machine presented to the guest, meaning that just because a host has a
> particular hardware capability does not mean we should automatically
> present that capability to the guest.
>
> In this case, QEMU will want a way to control whether the guest sees
> the availability of the second DAWR/X registers or not, i.e. whether a
> H_SET_MODE to set DAWR[X]1 will succeed or fail.

Patch #3 adds new kvm capability KVM_CAP_PPC_DAWR1 that can be checked
by Qemu. Also, as suggested by David in Qemu patch[1], I'm planning to
add new machine capability in Qemu:

  -machine cap-dawr1=ON/OFF

cap-dawr1 will be default ON when PPC_FEATURE2_ARCH_3_10 is set and OFF
otherwise.

Is this correct approach?

[1]: https://lore.kernel.org/kvm/20200724045613.GA8983@xxxxxxxxxxxxxxx

Thanks,
Ravi