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Re: [Qemu-devel] [PATCH] accel: Improve selection of the default acceler
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator |
Date: |
Tue, 9 Oct 2018 17:35:44 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0 |
On 09/10/2018 16:23, Daniel P. Berrangé wrote:
>>>
>>> (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".
... and because we rely on people that want KVM using almost always the
distro qemu-kvm.
> Putting KVM first shouldn't break existing working command lines
> in general.
>
> If user doesn't have KVM it won't have any impact as it'll still
> fallback to TCG. If user does have KVM, their VMs will be faster.
>
> It could conceivably break if
>
> - TCG had some different behaviour to KVM that
> the guest relied upon (we should fix such differences)
TCG can emulate stuff that doesn't exist in the host processor (e.g.
MPX, PKRU, and on older processors SMEP/SMAP though those should be
ubiquitous). There are (or were) CI systems for the kernel that were
using it that way.
Paolo
> - KVM kmod was buggy (eg broken nested virt, again should be
> something we fix)
> - If the guest OS relied on things being slow (that's already
> fragile)
>
> IMHO we should assume that TCG & KVM are functionally equivalent
> and non-broken, and thus changing default accelerator should not
> be considered an incompatible change.
- Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator, (continued)
- Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator, Markus Armbruster, 2018/10/09
- Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator, Markus Armbruster, 2018/10/09
- Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator, Thomas Huth, 2018/10/09
- Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator, Daniel P . Berrangé, 2018/10/09
- Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator, Cornelia Huck, 2018/10/09
- Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator, Peter Maydell, 2018/10/09
- Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator, Daniel P . Berrangé, 2018/10/09
- Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator, Peter Maydell, 2018/10/09
- Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator, Cornelia Huck, 2018/10/09
- Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator,
Paolo Bonzini <=
Re: [Qemu-devel] [PATCH] accel: Improve selection of the default accelerator, Philippe Mathieu-Daudé, 2018/10/05