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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.




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