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

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Message-ID: <34fc0944-92af-cd59-f1e4-26f351f42e70@linux.ibm.com>
Date:   Wed, 2 Sep 2020 10:39:06 +0530
From:   Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
To:     Paul Mackerras <paulus@...abs.org>
Cc:     mpe@...erman.id.au, mikey@...ling.org, npiggin@...il.com,
        pbonzini@...hat.com, christophe.leroy@....fr, jniethe5@...il.com,
        pedromfc@...ibm.com, rogealve@...ibm.com, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kselftest@...r.kernel.org,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Subject: Re: [PATCH 2/7] powerpc/watchpoint/kvm: Add infrastructure to support
 2nd DAWR

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