Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator

[Peter Maydell]

On 9 October 2018 at 15:23, Daniel P. Berrangé <address@hidden> wrote:
> On Tue, Oct 09, 2018 at 02:58:41PM +0100, Peter Maydell wrote:
>> On 9 October 2018 at 14:43, Cornelia Huck <address@hidden> wrote:
>> > I'm not sure why a single accelerator (any of them) would be a good
>> > default. A list (tcg:kvm:<whatever>) sounds much saner, as it would
>> > continue to work even if some accelerators have been disabled (right?)
>> >
>> > (And I'd prefer kvm to be first in that list; anything that relies on
>> > tcg being used should specify it explicitly... a normal user will
>> > likely always want the fast variant.)
>>
>> tcg should be the default for binaries without KVM compiled in,
>> of course... But as Thomas points out, the reason for our current
>> default is the usual "because we tend not to change things that
>> would break existing working command lines".
>
> Putting KVM first shouldn't break existing working command lines
> in general.

There are ARM QEMU command lines which will fail with KVM
and work with TCG (eg ones which use -cpu something-other-than-host
or which ask for a GICv3 when the host has only a GICv2).
These are basically cases where KVM can't provide features
that TCG can. I imagine other archs like power have similar.


thanks
-- PMM