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From: =?utf-8?Q?Llu=C3=ADs_Vilanova?= <vilanova@ac.upc.edu>
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Date: Thu, 31 Aug 2017 18:13:21 +0300
In-Reply-To: <20170831115935.GQ13619@stefanha-x1.localdomain> (Stefan
	Hajnoczi's message of "Thu, 31 Aug 2017 12:59:35 +0100")
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Subject: Re: [Qemu-devel] [PATCH v8 0/5] hypertrace: Lightweight
 guest-to-QEMU trace channel
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To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: "Emilio G. Cota" <cota@braap.org>, qemu-devel@nongnu.org, Luiz Capitulino <lcapitulino@redhat.com>

Stefan Hajnoczi writes:

> On Fri, Aug 25, 2017 at 08:34:54PM -0400, Emilio G. Cota wrote:
>> On Sun, Jul 30, 2017 at 17:08:18 +0300, Llu=C3=ADs Vilanova wrote:
>> > The hypertrace channel allows guest code to emit events in QEMU (the h=
ost) using
>> > its tracing infrastructure (see "docs/trace.txt"). This works in both =
'system'
>> > and 'user' modes, is architecture-agnostic and introduces minimal nois=
e on the
>> > guest.
>> >=20
>> > See first commit for a full description, use-cases and an example.
>> >=20
>> > Signed-off-by: Llu=C3=ADs Vilanova <vilanova@ac.upc.edu>
>>=20
>> This would be indeed very useful once TCG instrumentation is in place.
>>=20
>> However, I'm not very excited about this being PCI-only and Linux-only f=
or
>> system mode
>> I wonder how we could make this work on all hosts -- did you consider us=
ing
>> "magic" instructions? We'd need a different magic instruction for each
>> guest ISA, but the library would hide that anyway (and the library code
>> would be the same for user and system modes).

> Magic instructions can potentially be implemented more efficiently in
> TCG too.  They wouldn't work under KVM/HAX/hvf accelerators though.

Yes, we discussed this on the first version of this series long ago. The sp=
ecial
device makes it work on both TCG and KVM (if it were possible to implement a
TCG-to/from-KVM mode switch that'd be awesome).

Also, both approaches have negligible performance impact on the *guest* tim=
e.


Cheers,
  Lluis