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From: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: paulus@samba.org, mpe@ellerman.id.au, mikey@neuling.org,
	pbonzini@redhat.com, mst@redhat.com, clg@kaod.org,
	qemu-ppc@nongnu.org, qemu-devel@nongnu.org, kvm@vger.kernel.org,
	cohuck@redhat.com, groug@kaod.org,
	Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Subject: Re: [PATCH v5 3/3] ppc: Enable 2nd DAWR support on p10
Date: Wed, 21 Apr 2021 11:50:40 +0530
Message-ID: <ca21d852-4b54-01d3-baab-cc8d0d50e505@linux.ibm.com> (raw)
In-Reply-To: <YH0M1YdINJqbdqP+@yekko.fritz.box>

Hi David,

On 4/19/21 10:23 AM, David Gibson wrote:
> On Mon, Apr 12, 2021 at 05:14:33PM +0530, Ravi Bangoria wrote:
>> As per the PAPR, bit 0 of byte 64 in pa-features property indicates
>> availability of 2nd DAWR registers. i.e. If this bit is set, 2nd
>> DAWR is present, otherwise not. Use KVM_CAP_PPC_DAWR1 capability to
>> find whether kvm supports 2nd DAWR or not. If it's supported, allow
>> user to set the pa-feature bit in guest DT using cap-dawr1 machine
>> capability. Though, watchpoint on powerpc TCG guest is not supported
>> and thus 2nd DAWR is not enabled for TCG mode.
>> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
>> Reviewed-by: Greg Kurz <groug@kaod.org>
> So, I'm actually not sure if using an spapr capability is what we want
> to do here.  The problem is that presumably the idea is to at some
> point make the DAWR1 capability default to on (on POWER10, at least).
> But at that point you'll no longer to be able to start TCG guests
> without explicitly disabling it.  That's technically correct, since we
> don't implement DAWR1 in TCG, but then we also don't implement DAWR0
> and we let that slide... which I think is probably going to cause less
> irritation on balance.

Ok. Probably something like this is what you want?

Power10 behavior:
   - KVM does not support DAWR1: Boot the guest without DAWR1
     support (No warnings). Error out only if user tries with
   - KVM supports DAWR1: Boot the guest with DAWR1 support, unless
     user specifies cap-dawr1=off.
   - TCG guest: Ignore cap-dawr1 i.e. boot as if there is only
     DAWR0 (Should be fixed in future while adding PowerPC watch-
     point support in TCG mode)

Power10 predecessor behavior:
   - KVM guest: Boot the guest without DAWR1 support. Error out
     if user tries with cap-dawr1=on.
   - TCG guest: Ignore cap-dawr1 i.e. boot as if there is only
     DAWR0 (Should be fixed in future while adding PowerPC watch-
     point support in TCG mode)

> I'm wondering if we're actually just better off setting the pa feature
> just based on the guest CPU model.  TCG will be broken if you try to
> use it, but then, it already is.  AFAIK there's no inherent reason we
> couldn't implement DAWR support in TCG, it's just never been worth the
> trouble.

Correct. Probably there is no practical usecase for DAWR in TCG mode.


  reply index

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-12 11:44 [PATCH v5 0/3] ppc: Enable 2nd DAWR support on Power10 Ravi Bangoria
2021-04-12 11:44 ` [PATCH v5 1/3] Linux headers: update from 5.12-rc3 Ravi Bangoria
2021-04-12 11:44 ` [PATCH v5 2/3] ppc: Rename current DAWR macros and variables Ravi Bangoria
2021-04-19  4:47   ` David Gibson
2021-04-12 11:44 ` [PATCH v5 3/3] ppc: Enable 2nd DAWR support on p10 Ravi Bangoria
2021-04-12 13:10   ` Cédric Le Goater
2021-04-19  4:53   ` David Gibson
2021-04-21  6:20     ` Ravi Bangoria [this message]
2021-04-21  6:31       ` Cédric Le Goater
2021-04-21  6:54         ` Ravi Bangoria
2021-04-22  1:56           ` David Gibson
2021-04-22  4:45             ` Richard Henderson
2021-05-05  5:50       ` David Gibson

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