Hi Miguel, On 6/17/25 5:50 PM, Miguel Luis wrote: > Hi Eric, > >> On 17 Jun 2025, at 15:41, Eric Auger <eric.au...@redhat.com> wrote: >> >> >> >> On 6/17/25 5:23 PM, Miguel Luis wrote: >>> Hi Alyssa, >>> >>>> On 17 Jun 2025, at 14:17, Alyssa Ross <h...@alyssa.is> wrote: >>>> >>>> Eric Auger <eric.au...@redhat.com> writes: >>>> >>>>> From: Haibo Xu <haibo...@linaro.org> >>>>> >>>>> Up to now virt support on guest has been only supported with TCG. >>>>> Now it becomes feasible to use it with KVM acceleration. >>>>> >>>>> Also check only in-kernel GICv3 is used along with KVM EL2. >>>>> >>>>> Signed-off-by: Haibo Xu <haibo...@linaro.org> >>>>> Signed-off-by: Miguel Luis <miguel.l...@oracle.com> >>>>> Signed-off-by: Eric Auger <eric.au...@redhat.com> >>>>> Reviewed-by: Richard Henderson <richard.hender...@linaro.org> >>>> Hi! From what I can tell, this will produce an error on hosts that >>>> don't support nested virtualization when QEMU is invoked with -accel >>>> kvm:tcg >>> I didn’t know '-acell kvm:tcg’ could be used as a fallback mechanism between >>> acceleration modes. May I ask whether do you manage the ‘-cpu’ type for >>> ‘-accel >>> kvm:tcg’ with cpu ‘max’ ? >> Does it exist? >> qemu-system-aarch64: -accel kvm:tcg: invalid accelerator kvm:tcg > Maybe Alyssa is referring to ‘-M > virt,accel=kvm:tcg,virtualization=on,gic-version=3’ ? > > The above didn’t triggered any error. Anyhow if the above does what Alyssa’s > saying > we would just be missing the check for || !tcg_enabled() in this patch, I > believe. You meant you would report an error only if !tcg_enabled()?
In practice tcg_enabled() does not seem to be set when trying kvm first. Refering to the ancestor of this feature - which I did not know I acknowledge-, https://patchwork.ozlabs.org/project/qemu-devel/patch/1275954730-8196-20-git-send-email-aligu...@us.ibm.com/ it is said: kvm|tcg tries kvm and fallbacks on tcg if kvm support isn't available It does not state that each option is evaluated with kvm_enabled() and in case it fails enabling one because the host does not support it, it falls back to TCG. I checked for instance with already integrated MTE option and if the host does not support it, it does not offer the capability to fall back to TCG. Adding Peter in TO to get his feeling on this. Thanks Eric Besides > > Miguel > >> Alyssa, didn't you mean -accel kvm or --accel tcg >>> But more importantly, is this what you’re referring to? >>> >>> Although, >>> >>>> -machine virtualization=on, >>> should work for both '-accel kvm’ and ‘-accel tcg’. >>> >>>> but I don't think that's the ideal >>>> behaviour. It would make more sense for it to fall back to the first >>>> permitted accel option that does support running the machine as >>>> configured, so if hardware nested virtualization is not supported, it >>>> should fall back to TCG. >>>> >>>> I maintain an OS development environment that includes scripts for >>>> running images in QEMU, where running KVM on those images is a >>>> requirement. Currently, those scripts simply force TCG on aarch64. >>>> With this change, to take advantage of KVM NV support, I'd have to try >>>> to identify in the script whether NV would be supported. QEMU would be >>>> in a much better position to determine this and fall back to TCG if it's >>>> unsupported, like how the -accel option with multiple values usually >>>> works. >>> Thanks, >>> Miguel >
