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([2a01:e0a:466:71c0:8d4b:4003:dcb:4c70]) by smtp.gmail.com with ESMTPSA id v8sm729157wrw.2.2020.01.07.09.43.31 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 07 Jan 2020 09:43:31 -0800 (PST) Mime-Version: 1.0 (Mac OS X Mail 13.0 \(3608.40.2.2.4\)) Subject: Re: Priority of -accel (was: [PATCH] tests/qemu-iotests: Update tests to recent desugarized -accel option) From: Christophe de Dinechin In-Reply-To: <360fa010-ba80-b02b-3a35-19c2b48a462d@redhat.com> Date: Tue, 7 Jan 2020 18:43:30 +0100 Message-Id: <5FB9F11E-77DC-4FD6-B780-AB508DD42B42@redhat.com> References: <20200106130951.29873-1-philmd@redhat.com> <12334054-4ae7-e580-9727-2d322bfa2bda@redhat.com> <1A5859EA-4403-4921-B527-DFD07C59C702@redhat.com> <360fa010-ba80-b02b-3a35-19c2b48a462d@redhat.com> To: Paolo Bonzini X-Mailer: Apple Mail (2.3608.40.2.2.4) X-MC-Unique: legcxuitNGaZUJ2EZSNojw-1 X-Mimecast-Spam-Score: 0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 205.139.110.61 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Thomas Huth , Daniel Berrange , qemu-block@nongnu.org, qemu-devel@nongnu.org, Wainer dos Santos Moschetta , Max Reitz , =?utf-8?Q?Philippe_Mathieu-Daud=C3=A9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" > On 7 Jan 2020, at 15:37, Paolo Bonzini wrote: >=20 > On 07/01/20 14:55, Christophe de Dinechin wrote: >> So what about ranking the accelerators, so that all combinaisons >> -accel=3Dkvm:tcg, -accel=3Dtcg:kvm, -accel kvm -accel tcg, etc would >=20 > (I assume you mean "-machine accel=3Dkvm:tcg" and "-machine accel=3Dtcg:k= vm" > for the first two. This is the "older" way which has now become sugar > for "-accel kvm -accel tcg"). Yes. >=20 >> all pickup kvm if available, and tcg as a fallback? Implementation-wise, >> it would simply mean ranking the accelerators and updating the accelerat= or >> only if it=E2=80=99s available and better. >=20 > This is an interesting idea. I like this better than "-accel best", > because "-accel best" has the problem that you can't add suboptions to > it (the suboptions for the various accelerators are disjoint). >=20 > It would break backwards compatibility for "-machine accel=3Dtcg:kvm", > which so far meant "use TCG if compiled in, otherwise use KVM". This is > not something I would have a problem with... except that "tcg:kvm" is > the default if no -accel option is provided! What is the rationale for picking tcg over kvm? My guess is that when this was selected, KVM was the new fancy unstable thing and this was deemed the safe choice. My other guess is that this was around 1907 or so :-) My third guess is that you will probably provide me with a much better rationale ;-) Without knowing a rationale, my mind goes wild. I=E2=80=99m trying to imagi= ne who runs qemu directly, without the -accel option. For some reason, I picture a guy named Joe typing something like "qemu win95.qcow2=E2=80=9D, somewhere in a dark and dusty basement with old neon lamps flashing in the background and a few rats gnawing at damp CAT5 cables. Joe is intent on playing Day of the Tentacle one last time before it=E2=80= =99s too late. Joe's only complaint so far has been that the game was a bit slow, and that=E2=80=99s why he=E2=80=99s nervous and sweats profusely. Will Joe have the time to complete this one last task? Now, it turns out that Joe just updated his system, and suddenly, to his amazement, Day of the Tentacle is fluid and fast. It=E2=80=99s a miracle! What is Joe going to do that might be an issue for us? Maybe Joe will immediately run to Twitter, eyes full of thankful tears, full of a burning desire to tell the entire world that the qemu folks went totally overboard with this release and super-optimized it like crazy? (Consider the above as my way to tell that I=E2=80=99m a bit puzzled as to = why selecting kvm would be a problem. I even intentionally inserted win95, hoping that this might be one of the scenarios where kvm might be more trouble than anything, but I don=E2=80=99t know that for sure, having not t= ested it)