Re: [PATCH 2/2] x86/kvm: don't forget to ACK async PF IRQ

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Ingo Molnar <mingo@kernel.org> writes:

> * Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
>> Merge commit 26d05b368a5c0 ("Merge branch 'kvm-async-pf-int' into HEAD")
>> tried to adapt the new interrupt based async PF mechanism to the newly
>> introduced IDTENTRY magic but unfortunately it missed the fact that
>> DEFINE_IDTENTRY_SYSVEC() doesn't call ack_APIC_irq() on its own and
>> all DEFINE_IDTENTRY_SYSVEC() users have to call it manually.
>> 
>> As the result all multi-CPU KVM guest hang on boot when
>> KVM_FEATURE_ASYNC_PF_INT is present. The breakage went unnoticed because no
>> KVM userspace (e.g. QEMU) currently set it (and thus async PF mechanism
>> is currently disabled) but we're about to change that.
>> 
>> Fixes: 26d05b368a5c0 ("Merge branch 'kvm-async-pf-int' into HEAD")
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>
> This also fixes a kvmtool regression, but interestingly it does not set 
> KVM_FEATURE_ASYNC_PF_INT either AFAICS:
>
>   kepler:~/kvmtool.git> git grep KVM_FEATURE_ASYNC_PF_INT
>   kepler:~/kvmtool.git> 

My wild guess would be that kvmtool doesn't manually set any of the KVM
PV features:

[vitty@vitty kvmtool]$ git grep KVM_FEATURE_
[vitty@vitty kvmtool]$ 

it just blindly passes whatever it gets from KVM via
KVM_GET_SUPPORTED_CPUID to KVM_SET_CPUID2 and KVM_FEATURE_ASYNC_PF_INT
among other PV features is set there by default.

>
>   kepler:~/kvmtool.git> grep url .git/config
> 	url = https://git.kernel.org/pub/scm/linux/kernel/git/will/kvmtool.git
>
> So either I missed the flag-setting in the kvmtools.git source, or maybe 
> there's some other way to trigger this bug?
>
> Anyway, please handle this as a v5.9 regression:
>
> 	Tested-by: Ingo Molnar <mingo@kernel.org>

Thanks!

-- 
Vitaly