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Subject: Re: [PATCH 2/7] powerpc/watchpoint/kvm: Add infrastructure to support
 2nd DAWR
To:     Paul Mackerras <paulus@ozlabs.org>
Cc:     mpe@ellerman.id.au, mikey@neuling.org, npiggin@gmail.com,
        pbonzini@redhat.com, christophe.leroy@c-s.fr, jniethe5@gmail.com,
        pedromfc@br.ibm.com, rogealve@br.ibm.com, kvm@vger.kernel.org,
        kvm-ppc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-kselftest@vger.kernel.org,
        Ravi Bangoria <ravi.bangoria@linux.ibm.com>
References: <20200723102058.312282-1-ravi.bangoria@linux.ibm.com>
 <20200723102058.312282-3-ravi.bangoria@linux.ibm.com>
 <20200902020124.GB272502@thinks.paulus.ozlabs.org>
From:   Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Message-ID: <34fc0944-92af-cd59-f1e4-26f351f42e70@linux.ibm.com>
Date:   Wed, 2 Sep 2020 10:39:06 +0530
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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@umbus.fritz.box

Thanks,
Ravi